At 17, Dwight Grandson should have been finishing up his studies at Mount Vernon High School. Instead, he went to prison for murder.

Last year, after nearly six years behind bars, Grandson was granted a new trial. This month, a second D.C. Superior Court jury decided Grandson, 24, was not guilty.

Armed with an equivalency diploma earned in a Virginia penitentiary and wearing makeup to cover his tattoos, he speaks of plans to restart his education and of a career counseling at-risk youths.

Grandson smiled during a recent interview in his attorney’s office. “It was unbelievable,” he said after a three-day retrial that ended after fewer than three hours of deliberations. “In just 21 / 2 hours, I got my life back.”

Grandson’s reprieve resulted from a prosecutor’s mistake in failing to reveal key information about government witnesses and what they knew.

Although Grandson said he is pleased to have his life ahead of him, defense lawyers say that what happened is part of a pattern in the District, a charge the city’s top prosecutor denies. Still, there have been several recent examples of evidence violations leading to case dismissals and retrials, as judges have agreed with defense lawyers that rules were broken.

Prosecutors must hand their evidence to the defense at the earliest possible time, even if that evidence weakens their case. Failure or a delay to do so can be considered misconduct and a violation of a 1963 rule based on the Supreme Court case Brady v. Maryland.

Defense lawyers have long complained that some prosecutors in the U.S. attorney’s office hamper their investigations by withholding evidence or keeping it until the last minute. Prosecutors say such complaints are tactics to try to delay trials.

Grandson was convicted of fatally shooting Derrick Henson in Southeast Washington in 2004. D.C. Superior Court Judge Rhonda Reid-Winston ordered a retrial, writing in a 47-page ruling that the prosecutor made “repeated, blatant Brady violations and misrepresentations.”

Reid-Winston identified eight examples of violations, including the prosecutor’s denial and subsequent admission that he knew a key witness sought a $25,000 reward for her testimony. The judge said the prosecutor also failed to reveal that a witness told a grand jury that Grandson regularly carried a .25-caliber handgun; the victim was shot with a .45-caliber semi-automatic.

Reid-Winston further cited the prosecutor’s delays in turning over evidence that a witness was on medication for schizophrenia and smoked PCP; in releasing a witness’s medical records despite court orders to do so; and in releasing a recorded telephone call the victim made after the shooting.

Finally, according to Reid-Winston’s ruling, the prosecutor kept a police recording — in which a witness described the shooter to an officer — for 18 months, giving it to the defense on the day Grandson’s trial was scheduled to begin.

In a statement from U.S. Attorney Ronald C. Machen Jr.’s office, prosecutors said much of the problem in Grandson’s case centered on timing of disclosures, not the failure to disclose information.

“Our office is committed to broad and prompt disclosure, but the timing of disclosures is often a difficult issue in violent crime cases, because prosecutors not only have different perspectives from defense attorneys about the sort of information that could potentially be deemed exculpatory, but also have to be concerned about witness safety, which can be put in jeopardy if defendants learn the identity of government witnesses and how those witnesses plan to testify before trial,” the office said in a statement.

Machen’s office said Grandson’s case needs to be taken in “context” and not as a representation of the thousands of cases handled by its prosecutors. The statement said his office takes the Brady rule seriously. “The mission of this office is to do justice and enhance public safety — something this office has a long and proud tradition of achieving on behalf of the District of Columbia’s residents — not simply obtain convictions at any cost.”

Machen, in a separate statement, said that Brady violations are rare and that when such violations are discovered, they are acknowledged and dealt with.

“The U.S. attorney’s office — along with the rest of the Justice Department — has taken far-reaching steps to ensure that we try to meet and exceed our discovery obligations in every case,” Machen said. “It is because of the integrity of our prosecutors and support staff and our rigorous training that errors are exceedingly rare.”

Prosecutorial disclosure

When Judge Richard C. Tallman, then-head of the federal Judicial Conference Advisory Committee, conducted a national review of prosecutorial disclosure last year, the District’s Public Defender Service identified Grandson’s case and four others in a memo. According to the document:

●Judge Frederick H. Weisberg wrote that a prosecutor in the 2009 murder trial of Joseph Harrington “withheld from the defense consciously, deliberately and as a tactic.”

Harrington was charged with first-degree murder. Weisberg determined that the prosecutor had withheld a witness’s identity, statements to police and grand jury testimony that contradicted a key government witness. Harrington pleaded guilty to second-degree murder before a retrial was scheduled to begin.

●In 2008, Judge Brian Holeman dismissed a robbery and simple assault case against Theresa Green after learning that the prosecutor had waited 18 months to disclose information from the alleged victim that the judge had ruled was beneficial to Green.

Holeman also learned that the prosecutor delayed revealing that a police report had not been taken because the alleged victim was intoxicated and gave police three accounts of the robbery.

●Judge Wendell P. Gardner dismissed charges of assault on a police officer against Leonardo Delacruz in 2008. Gardner issued the ruling after learning that the prosecutor had waited until closing arguments to reveal a witness’s statement supporting the defense’s case.

●In 2006, Judge Erik Christian granted a mistrial in a second-degree murder case against Antonio Linder after Christian learned that the prosecutor failed to disclose statements from a key witness who second-guessed the identification he gave authorities and a grand jury. In 2007, a jury acquitted Linder after a three-day retrial.

Machen, who served as an assistant U.S. attorney in the District from 1997 to 2001, worked at a private law firm and was not affiliated with the prosecutor’s office when those five cases were pursued. Machen was appointed U.S. attorney in early 2010.

“You have to remember that the errors identified make up just 1/100th of 1 percent of the total number of criminal cases we handle,” Machen said in his statement. “The truth is that every year the small number of errors that prosecutors make are overwhelmed in number by the scores of frivolous allegations of prosecutorial misconduct that are subsequently determined to be baseless by the courts.”

Little change

A year after the memo was sent to Tallman, criminal defense lawyers say they have seen little change.

“The government continues to fall short of its constitutional obligation to provide exculpatory evidence to the defense in time for the defense to investigate it and present it to the jury at trial,” said Julia Leighton, general counsel for the Public Defender Service. “This ongoing failure undermines the fairness of trials.”

Cases continue to be affected. In March, in a 2-to-1 decision, a three-judge panel of the D.C. Court of Appeals granted a mistrial in a shooting case against Tyrone B. Miller on the basis of Brady violations.

“A prosecutor’s timely disclosure obligation with respect to Brady material can never be overemphasized,” Judge Frank Schwelb wrote in a 44-page decision. Miller’s retrial is pending.

Tallman and his committee ruled in May that changes to disclosure regulations weren’t needed but said that judges should have more training in spotting possible issues before a trial.

“No rule can effectively prevent intentional misconduct by prosecutors who knowingly withhold exculpatory information,” Tallman wrote in his report. “But the committee was not convinced that the problem is so severe as to warrant a rule change when existing Supreme Court authority on a prosecutor’s disclosure obligations is clear and for which substantial sanctions are available for non-compliance.”

Prosecutors who falter can be subject to disciplinary actions and referral to the state bar. It was unclear how many prosecutors in the District have been sanctioned or investigated by Justice Department officials. A Justice spokeswoman said that information was private, citing personnel issues.

Grandson’s attorney, Kevin L. Mosley, said the decision about what and when to release to the defense should be taken out of prosecutors’ hands and given to judges — even the information that, as prosecutors say, could jeopardize the safety of a witness. “There’s too much discretion in the hands of individual prosecutors,” Mosley said.

Life after prison

Grandson and Mosley declined to discuss Henson’s killing and Grandson’s arrest. Now living with his father, Grandson said he no longer ventures into Southeast Washington.

After he was charged in the shooting, Grandson was housed in the juvenile area of the D.C. jail. After his sentencing two years later, he was sent to a penitentiary in Lee County, Va.

Prison, Grandson said, was “no place to be a kid. I was respectful, and I minded my business.

“I grew up quick.”