The prosecutor had another explanation: Leiterman killed the student. He was linked to the crime by other evidence, including a handwriting sample and a gun he owned that was similar to the murder weapon.
The prosecutor admitted that the 4-year-old could not have been the killer but speculated that he might also have been at the crime scene, with a nose bleed. A jury convicted Leiterman of first-degree murder. He is appealing the verdict.
Fingering the wrong suspect can be a sign of broader sloppiness in a lab.
A Sacramento crime lab analyst had to resign two years ago after his superiors discovered he had relied on substandard DNA evidence in a rape case to produce a profile that matched an innocent man. The lab later learned the analyst had failed to check his work in dozens of other cases.
But even the most scrupulous analyst can err when interpreting complex DNA "mixtures" -- samples that contain DNA from more than one person -- which turn up more frequently as labs use more sensitive tests.
"If you show 10 colleagues a mixture, you will probably end up with 10 different answers," Peter Gill, then a chief scientist with Britain's Forensic Science Service, said at a symposium in 2005.
In a 2005 study, American laboratories that analyzed the exact same DNA mixture reported widely different statistics on the rarity of the genetic profiles of the contributors.
Estimates of how often the DNA profiles occurred in the general population ranged from 1 in 400,000 people to 1 in 1,000,000,000,000,000.
In the courtroom, say some leaders in the field, the overwhelming numbers presented to jurors convey a near-certainty the science does not justify.
Serious backlog
As much as errors can hurt the wrongfully accused, victims don't see justice served when DNA evidence goes untested.
The nation's labs have a serious backlog of DNA crime-scene samples awaiting analysis, as many as 400,000 cases involving rapes and other violent crimes, according to Human Rights Watch.
Meanwhile, criminals are free to strike again: In Ohio, a man committed 13 more rapes while his DNA sat inside a rape kit in storage for two years.
Earlier this year the Los Angeles Police Department acknowledged a backlog of as many as 7,000 DNA crime samples. The Los Angeles County Sheriff's Department later admitted to having 4,727 untested sexual assault evidence kits.
Law enforcement officials blamed a lack of money and manpower. But the federal government contends that the LAPD and other agencies aren't spending all the money they allocate for backlog reduction. For that very reason, it cut LAPD's $1 million share of the funding in half this year.
"We have the science, we have the technology, we have the capacity to prevent rapes. And we are not using it to its fullest potential," said Gail Abarbanel, director of the Rape Treatment Center at Santa Monica-UCLA Medical Center.
Even when crime scene evidence is analyzed and a suspect identified, authorities don't always pursue conviction.
Nationally, the FBI reported hits in the national database aided 77,791 investigations this year, but no one tracks how many result in convictions.
"Hundreds of DNA database hits languish without any follow up by law enforcement or prosecutors," Frederick Bieber, a Harvard Medical School professor, reported in the journal American Society of Law, Medicine & Ethics, in 2006.
In June 2004, a DNA profile from a sexual assault in Oakland matched that of a convicted child molester. By the time police contacted the man six months later, he had molested another child.
The prosecutor admitted that the 4-year-old could not have been the killer but speculated that he might also have been at the crime scene, with a nose bleed. A jury convicted Leiterman of first-degree murder. He is appealing the verdict.
Fingering the wrong suspect can be a sign of broader sloppiness in a lab.
A Sacramento crime lab analyst had to resign two years ago after his superiors discovered he had relied on substandard DNA evidence in a rape case to produce a profile that matched an innocent man. The lab later learned the analyst had failed to check his work in dozens of other cases.
But even the most scrupulous analyst can err when interpreting complex DNA "mixtures" -- samples that contain DNA from more than one person -- which turn up more frequently as labs use more sensitive tests.
"If you show 10 colleagues a mixture, you will probably end up with 10 different answers," Peter Gill, then a chief scientist with Britain's Forensic Science Service, said at a symposium in 2005.
In a 2005 study, American laboratories that analyzed the exact same DNA mixture reported widely different statistics on the rarity of the genetic profiles of the contributors.
Estimates of how often the DNA profiles occurred in the general population ranged from 1 in 400,000 people to 1 in 1,000,000,000,000,000.
In the courtroom, say some leaders in the field, the overwhelming numbers presented to jurors convey a near-certainty the science does not justify.
Serious backlog
As much as errors can hurt the wrongfully accused, victims don't see justice served when DNA evidence goes untested.
The nation's labs have a serious backlog of DNA crime-scene samples awaiting analysis, as many as 400,000 cases involving rapes and other violent crimes, according to Human Rights Watch.
Meanwhile, criminals are free to strike again: In Ohio, a man committed 13 more rapes while his DNA sat inside a rape kit in storage for two years.
Earlier this year the Los Angeles Police Department acknowledged a backlog of as many as 7,000 DNA crime samples. The Los Angeles County Sheriff's Department later admitted to having 4,727 untested sexual assault evidence kits.
Law enforcement officials blamed a lack of money and manpower. But the federal government contends that the LAPD and other agencies aren't spending all the money they allocate for backlog reduction. For that very reason, it cut LAPD's $1 million share of the funding in half this year.
"We have the science, we have the technology, we have the capacity to prevent rapes. And we are not using it to its fullest potential," said Gail Abarbanel, director of the Rape Treatment Center at Santa Monica-UCLA Medical Center.
Even when crime scene evidence is analyzed and a suspect identified, authorities don't always pursue conviction.
Nationally, the FBI reported hits in the national database aided 77,791 investigations this year, but no one tracks how many result in convictions.
"Hundreds of DNA database hits languish without any follow up by law enforcement or prosecutors," Frederick Bieber, a Harvard Medical School professor, reported in the journal American Society of Law, Medicine & Ethics, in 2006.
In June 2004, a DNA profile from a sexual assault in Oakland matched that of a convicted child molester. By the time police contacted the man six months later, he had molested another child.
