(This June 7 story corrects national tally of exonerations in fifth paragraph.)

(Reuters) - A Michigan court threw out the convictions and sentence of a man who was 14 when he pleaded guilty to a 2007 quadruple homicide in Detroit that a professional hit man later confessed to committing, prosecutors and defense attorneys said on Tuesday.

Davontae Sanford's sentence of up to 90 years was vacated by Wayne County Judge Brian Sullivan, according to statements from the Dykema Gossett law firm, which handled Sanford's appeal, and the Wayne County Prosecutor's Office, which also moved to overturn the conviction after a reinvestigation.

After being arrested in 2007, Sanford gave an inaccurate confession to the killings and pleaded guilty to four counts of second-degree homicide on the advice of an attorney now suspended from the practice of law, the Dykema Gossett statement said.

"In April 2008, two weeks after Sanford's sentencing, a professional hit man, Vincent Smothers, confessed to the Runyon Street killings and eight additional murders," the statement from the law firm said.

More than 1,810 exonerations have been recorded in the United States since 1989, for murder and other crimes, and the pace has increased steadily over the years to about three a week, according to the National Registry of Exonerations run by the University of Michigan Law School.

The trend has been driven both by so-called innocence projects run by law schools and by special units established within prosecutors' offices to examine possible false confessions and official misconduct.

In the Detroit case, the judge ordered prosecutors to file a motion to dismiss the case, which means Sanford will not be retried, said a statement from Wayne County Prosecutor Kym Worthy's office.

State police reinvestigated the case and found that a deputy who originally gave sworn testimony that Sanford drew an accurate diagram of the crime scene, later contradicted that testimony, the prosecutor's office's statement said.

The Michigan Innocence Clinic and other groups that work to overturn wrongful convictions had appealed the conviction, arguing that Smothers' confession matched the details of what happened in the murders, while Sanford's did not.

(Reporting by Fiona Ortiz in Chicago; Editing by Peter Cooney)