It's an unspoken truth that sometimes, people just vanish. They disappear so utterly that it almost feels like alien abduction. They leave only a few meagre possessions behind; clothes that stay on hangers, a bed that will never be slept in, and the same few photographs resting on shelves which remain ageless until the individual, or their mortal remains, are finally found. For their loved ones, a missing person's case is like an open wound that's never allowed to close.

Any investigator worth their salt will tell you that the longer a case goes on for, the harder it is to solve. But sometimes, a miracle occurs. Sometimes a new facet of the long-cold case will reveal itself and lead us to the truth. However, an ordinary dollar bill could well have given investigators a hint as to the final whereabouts of a little girl who went missing nearly two decades ago.

Saturday, January 2, 1999. America had finally managed to crawl out of its New Year's hangover, and in the midwest a sudden drop in temperature, combined with high winds, led to a brutal snowstorm. In sunny Arizona however, the climate was warm but not sweltering, and most families were enjoying the end of the precious holidays. In the suburb of Mesa, 20 miles east of Arizona, two little girls were playing in the street. They were waiting for the ice cream truck to pass by, tinkling its merry tune. One of them would never be seen again.

Their names were Kimber and Mikelle Biggs. Kimber, the younger sibling, stepped inside their house to speak to her mother. She was gone no more than 90 seconds, but 90 seconds was all it took. When she returned to the sidewalk outside the family home, she discovered Mikelle's bicycle lying abandoned in the street, along with the two quarters she'd clutched in her hand to pay for their ice cream. Mikelle Biggs, 11, was gone. This year, if she is indeed still alive, she will turn 30.

The Biggs family contacted the police, who immediately organised a search. Within an hour, a helicopter was hovering above Mesa and shining a spotlight over the town. At the time, it was the Mesa police precinct's biggest ever investigation. Local law enforcement collected over 800 pieces of evidence and received over 10,000 tips. They conducted 500 interviews with psychics and searched 35 abandoned mine shafts in the nearby San Tan Mountains. Detectives tracked every ice-cream van in the state, and for a time the state of Arizona was inundated with pictures of the smiling fifth-grader. But they had no witnesses, no conclusive evidence, no suspects and, crucially, no little girl. If Mikelle was still alive, she was far away from Mesa.

Without new information, the case stagnated. But then, in March, something new came to the attention of police. In the town of Neenah, Wisconsin, 2,338 kilometres away from the spot where Mikelle Biggs vanished, a dollar bill from 2009 offered the tantalising possibility of answers. Written along the edges of the banknote was the following message: "My name is Mikel  Biggs kidnapped From Mesa AZ I'm Alive [sic]." If the message was a joke, then it was in poor taste. If it was a hoax, then it was a cruel one. But if the message is legitimate and does indeed come from the real Mikelle, then perhaps the misspelled signature on the dog-eared bill could lead us closer to discovering what happened to her.

The message does appear to be written in a child's script, and handwriting experts have observed that, although Mikelle's name is spelt wrong, the "kel" in the name is written in cursive, which appears to suggest that it could be authentic. However, it's almost impossible to determine how this bill ended up in Neenah or trace it back to Mikelle herself. Investigator Adam Streubel is sceptical. "None of us were really aware of the kidnapping," he stated in a recent interview. "We had to do some research. There was a little spring of hope for a second, and then reality set in." However, Streubel is now collaborating with Mesa investigators, who are following up on the clue.

Mikelle's sister Kimber runs the Facebook page Justice for Mikelle Biggs. Kimber is also cautious about treating the note with more significance than is necessary. She believes that the misspelling indicates that it likely isn't from Mikelle, and could be a mean-spirited prank. That said, she is still hopeful. In a Facebook post written when she was first made aware of the dollar bill, Kimber wrote: "I was not notified by the detective or writers till after the fact. Not sure what to think. I'm honestly numb. Thank you for everyone who is reaching out."

In a later interview Kimber stated: "I think it could be a step in the right direction, at best ... I don't believe she would have written it, as the circumstances of it don't make much sense. There's always that bit of hope, but I think right now we just want it to lead to someone who knows something." Mesa police spokesperson Detective Steve Berry followed up by saying: "We don't get a lot of tips any more, but we occasionally do. We always follow up on it. We always hope that might be the one that breaks the case."

The last prime suspect in the Biggs case was convicted rapist Dee Blalock, who is currently serving a multiple life sentence at the Florence maximum security prison for the brutal sexual assault of neighbour Susan Quinnett. Quinnett was so convinced that Blalock was responsible for Mikelle's disappearance that she was willing to drop all charges against him if he would confess to the crime. Mikelle's parents Tracy and Darien met with Blalick in 2009, but he denied any involvement. If you or anyone you know has any information about Mikelle Biggs' whereabouts, please contact the Arizona Police Department on their website or call 480 644-2211.

It's an unspoken truth that sometimes, people just vanish. They disappear so utterly that it almost feels like alien abduction. They leave only a few meagre possessions behind; clothes that stay on hangers, a bed that will never be slept in, and the same few photographs resting on shelves which remain ageless until the individual, or their mortal remains, are finally found. For their loved ones, a missing person's case is like an open wound that's never allowed to close.

Any investigator worth their salt will tell you that the longer a case goes on for, the harder it is to solve. But sometimes, a miracle occurs. Sometimes a new facet of the long-cold case will reveal itself and lead us to the truth. However, an ordinary dollar bill could well have given investigators a hint as to the final whereabouts of a little girl who went missing nearly two decades ago.

Saturday, January 2, 1999. America had finally managed to crawl out of its New Year's hangover, and in the midwest a sudden drop in temperature, combined with high winds, led to a brutal snowstorm. In sunny Arizona however, the climate was warm but not sweltering, and most families were enjoying the end of the precious holidays. In the suburb of Mesa, 20 miles east of Arizona, two little girls were playing in the street. They were waiting for the ice cream truck to pass by, tinkling its merry tune. One of them would never be seen again.

Their names were Kimber and Mikelle Biggs. Kimber, the younger sibling, stepped inside their house to speak to her mother. She was gone no more than 90 seconds, but 90 seconds was all it took. When she returned to the sidewalk outside the family home, she discovered Mikelle's bicycle lying abandoned in the street, along with the two quarters she'd clutched in her hand to pay for their ice cream. Mikelle Biggs, 11, was gone. This year, if she is indeed still alive, she will turn 30.

The Biggs family contacted the police, who immediately organised a search. Within an hour, a helicopter was hovering above Mesa and shining a spotlight over the town. At the time, it was the Mesa police precinct's biggest ever investigation. Local law enforcement collected over 800 pieces of evidence and received over 10,000 tips. They conducted 500 interviews with psychics and searched 35 abandoned mine shafts in the nearby San Tan Mountains. Detectives tracked every ice-cream van in the state, and for a time the state of Arizona was inundated with pictures of the smiling fifth-grader. But they had no witnesses, no conclusive evidence, no suspects and, crucially, no little girl. If Mikelle was still alive, she was far away from Mesa.

Without new information, the case stagnated. But then, in March, something new came to the attention of police. In the town of Neenah, Wisconsin, 2,338 kilometres away from the spot where Mikelle Biggs vanished, a dollar bill from 2009 offered the tantalising possibility of answers. Written along the edges of the banknote was the following message: "My name is Mikel  Biggs kidnapped From Mesa AZ I'm Alive [sic]." If the message was a joke, then it was in poor taste. If it was a hoax, then it was a cruel one. But if the message is legitimate and does indeed come from the real Mikelle, then perhaps the misspelled signature on the dog-eared bill could lead us closer to discovering what happened to her.

The message does appear to be written in a child's script, and handwriting experts have observed that, although Mikelle's name is spelt wrong, the "kel" in the name is written in cursive, which appears to suggest that it could be authentic. However, it's almost impossible to determine how this bill ended up in Neenah or trace it back to Mikelle herself. Investigator Adam Streubel is sceptical. "None of us were really aware of the kidnapping," he stated in a recent interview. "We had to do some research. There was a little spring of hope for a second, and then reality set in." However, Streubel is now collaborating with Mesa investigators, who are following up on the clue.

Mikelle's sister Kimber runs the Facebook page Justice for Mikelle Biggs. Kimber is also cautious about treating the note with more significance than is necessary. She believes that the misspelling indicates that it likely isn't from Mikelle, and could be a mean-spirited prank. That said, she is still hopeful. In a Facebook post written when she was first made aware of the dollar bill, Kimber wrote: "I was not notified by the detective or writers till after the fact. Not sure what to think. I'm honestly numb. Thank you for everyone who is reaching out."

In a later interview Kimber stated: "I think it could be a step in the right direction, at best ... I don't believe she would have written it, as the circumstances of it don't make much sense. There's always that bit of hope, but I think right now we just want it to lead to someone who knows something." Mesa police spokesperson Detective Steve Berry followed up by saying: "We don't get a lot of tips any more, but we occasionally do. We always follow up on it. We always hope that might be the one that breaks the case."

The last prime suspect in the Biggs case was convicted rapist Dee Blalock, who is currently serving a multiple life sentence at the Florence maximum security prison for the brutal sexual assault of neighbour Susan Quinnett. Quinnett was so convinced that Blalock was responsible for Mikelle's disappearance that she was willing to drop all charges against him if he would confess to the crime. Mikelle's parents Tracy and Darien met with Blalick in 2009, but he denied any involvement. If you or anyone you know has any information about Mikelle Biggs' whereabouts, please contact the Arizona Police Department on their website or call 480 644-2211.