When three half brothers' decades-old murder convictions were thrown out last month, they became a dramatic example of an idea spreading among prosecutors nationwide: "integrity units" dedicated to double-checking convictions to determine whether justice was served.

Over the last seven years, more than a dozen prosecutors' offices across the country have created such staff teams or expert panels to review wrongful-conviction claims. The groups have agreed to revisit more than 4,900 cases, resulting in at least 61 convictions tossed so far, according to a tally compiled from interviews, prosecutors' reports and news accounts.

The initiatives are underway from New York City to Santa Clara County, California, and from Chicago to Dallas, fueled by growing concern about false convictions in a country that has averaged 68 exonerations a year since 1999.

"What we've seen take place over the last 15 years has, I think, shaken most career prosecutors to their core. . We had to respond to it," says Scott McNamara, district attorney in central New York's Oneida County. His year-old conviction review committee is working on three cases so far.

While advocates see the reviews as open-minded efforts to address possible injustice, some defense attorneys are cautious about embracing them. Some prosecutors have faced questions about whether they're out to rethink convictions or just harden them.

"It's a catchy name to say you're a conviction integrity unit, but you have to be able to answer some of these questions," said Barry Scheck, a co-founder of the Innocence Project, an exoneration advocacy group.

Police and prosecutors have long aided some exonerations without having special conviction-review units, and many still do. But since Dallas DA Craig Watkins started such a unit in 2007, it has freed 33 people, reviewed more than 400 cases and inspired other prosecutors. Philadelphia and Cleveland prosecutors followed suit this April.

Prosecutors acknowledge it's an unusual role for them.

"Why are we doing this?" some Santa Clara County prosecutors asked at first, recalls Assistant District Attorney David Angel, who oversees the effort there. His reply: If the criminal justice system errs, "we have to be part of reversing that error."

The scope and outcomes of the reviews vary.

Brooklyn prosecutors have disavowed seven murder convictions this year as they review more than 90 cases, while their colleagues in neighboring Manhattan have gotten four convictions dismissed in four years. Manhattan prosecutors have reassessed more than 150 cases and reinvestigated at least 12, but most claims "prove to be frivolous," DA Cyrus R. Vance Jr. said in 2012.

After a DNA-focused review of more than 1,700 cases by the Colorado attorney general and Denver DA's offices, one man was exonerated, though the work continues. Six convictions were overturned after the Detroit-based Wayne County Prosecutor's office retested gun evidence in more than 400 cases -- but all six defendants later pleaded guilty or were convicted at retrials.

Prosecutors note that the reviews aren't meant just to rehash. While reviewing convictions, "we also have to make sure we're respecting the integrity of the jury system," says Michael Nerheim, the state's attorney in suburban Lake County, Illinois. His year-old panel is poring over three or four cases.

Appeals courts have been re-examining convictions for centuries, but prosecutors' reviews can unearth evidence from authorities' own files that never made it into the court record. DAs also have investigative tools that defense lawyers don't, such as the ability to grant witnesses immunity from prosecution. And there can be "enormous advantage" in approaching prosecutors before going into the adversarial realm of court, says Claudia Trupp of the Center for Appellate Litigation in New York.

But other defense attorneys are skeptical.

Lawyer Ron Kuby branded the Manhattan DA's effort a "wrongful conviction perpetuation unit" last month after prosecutors rebuffed a prisoner's innocence claim in a notorious 1990 tourist killing. Prosecutors said they didn't believe what he'd presented, including a statement from a former co-defendant who was cleared.

Conviction review units in Brooklyn and Chicago-centered Cook County, Illinois, where reviews have led to six dismissals in two years, have faced complaints that they don't move fast enough. Prosecutors have noted the difficulty of reinvestigating decades-old cases.

Whatever the debate, it hardly mattered to Robert Hill when he left a Brooklyn court last month, free for the first time in 27 years, after prosecutors disavowed the 1980s convictions that had put him and his two half brothers behind bars.

"I'm just happy to be out," Hill said.