Barry Scheck, co-founder of the Innocence Project, a nonprofit group that fights for DNA exonerations, said most - about 120 of 160 exonerations since the advent of forensic DNA testing in 1989 - hinged on mistaken witness identification. But none involved as many mistaken witnesses as the Diaz case, he said.

"It's a landmark case for that reason," Mr. Scheck said. "These were crimes that upset people in the community, there was pressure to solve them, and there were, I think, unfortunate eyewitness techniques used that, in light of what we know now, were the kind that lead to error."

Scheck advocates sequential lineups. Showing witnesses one suspect at a time reduces the likelihood of mistaken identifications, countering the natural tendency to pick out the person in a group who most closely resembles the true culprit. But that technique only works if the officers conducting the lineup don't send clues to the witness, encouraging identification of the person they suspect is guilty. To avoid the introduction of bias, the officer conducting the lineup should have no idea which person in the lineup is the suspect, and the witness should be told that the suspect isn't necessarily in the lineup. Without these safeguards, this is what happens:

One victim, whose identification of Mr. Diaz led to his arrest, said she had originally thought none of the men in the nine photographs she was shown resembled her attacker. She had asked to see more pictures, she said, but settled on Mr. Diaz after the police twice told her to keep looking at the first batch. He was the only one who remotely looked like the attacker, she said.

The woman later picked Diaz out of a lineup, but at that point her memory had been tainted by having seen his picture repeatedly in the photo array. Realizing her error years later, the woman recanted.

Prosecutors concede that some victims may have been mistaken, but they refuse to acknowledge that Diaz is innocent of each of the rapes. In light of the shattered case against Diaz, however, they won't retry him. Joined by Diaz' defense lawyers, the prosecutors will ask a judge today to vacate Diaz' convictions -- 25 years after they sent him to prison for crimes he didn't commit.