Thirty-four of Prince George’s County’s most dangerous criminals sat side-by-side in black upholstered chairs at a Suitland health center one recent evening, staring suspiciously at the police commanders, prosecutors and community activists in front of them.

The murderers, robbers and carjackers had been forced to come to the meeting by their parole and probation agents. All are considered likely to strike again.

Officers will be watching closely this summer, police told the group. Those in the room who don’t stay out of trouble will go to prison.

“There will be no more, ‘Eh, let’s give it one more time,’” Prince George’s prosecutor C.T. Wilson warned.

Prince George’s authorities said they are appealing directly to convicts — a crime-fighting tactic catching on in police departments nationwide — to help stem violence in what has been an unusually deadly year.

Since Jan. 1, 56 people have been slain in the county, a dozen more than at the same time last year. The killings have stemmed from drug robberies, nightclub disputes and even mistaken identity. Many of the alleged killers and the victims had prior criminal records.

In hopes of heading off a spike in crime that typically comes in the hot summer months, police and prosecutors met face-to-face with offenders who live in five of Prince George’s most violent neighborhoods. Authorities threatened the convicts with the prospect of jail time, but also offered them job-training and counseling services. They urged them to help keep their own neighborhoods safe.

“The message is, ‘We don’t want you to re-offend,’” said Prince George’s Deputy Chief Kevin Davis, who is leading the effort. “Hopefully that will inspire some people to be on their best behavior over the summer.”

At the Suitland meeting — one of three in the county in recent weeks — most of the criminals were men, dressed in baggy shorts and T-shirts. Some slouched in their chairs. Others leaned forward with their elbows on their knees. One woman read a book.

Most seemed bored as authorities explained police would focus on their neighborhoods and prosecutors would seek tough punishment for offenders. Some, though, nodded along as Wilson, the prosecutor, described how carjackings cause car insurance rates to rise, how robberies deter pizza places from delivering to certain neighborhoods.

The criminals said little. One man asked how long he had to stick around, saying he was “on the clock.” Many darted for the elevator when police said they were free to go.

But some lingered afterward to talk with community activists who had offered job training, mentoring, counseling, even late night recreation center activities.

“I think that it was good,” said 48-year-old Cedric McCray, a convicted robber who was among those who stayed. “If I could go back to school and get my GED or do something like that, that would be good for me. I think we needed a wake up call.”

Law enforcement experts said the tactic, sometimes known as call-ins, works because a small group of people, often on parole and probation, are responsible for many violent crimes. Send the message to those people — authorities want to help you, but will not be lenient if you re-offend — and they will usually listen, said David M. Kennedy, the director of the Center for Crime Prevention and Control and a professor at the John Jay College of Criminal Justice in New York City.

“The record is really, really clear that they respect that and they respond to it,” Kennedy said. “What happens when you have these meetings is that word spreads like wildfire on the street.”

Police in High Point, N.C., for example, have seen a 47 percent drop in violent crime since implementing a version of the approach in 1997, said Marty Sumner, the deputy chief there. D.C. police are using call-ins this summer, having tried them in 2006 and 2010, authorities said. Of the 83 offenders at the 2010 call-in, 19 were caught committing crimes afterward — a figure officials considered “pretty successful,” said D.C. Assistant Chief Alfred Durham.

According to a recent Pew Center on the States study, 43.3 percent of prisoners who were released in 2004 were back behind bars within three years, based on data provided by 41 states. Maryland did not provide data in the form researchers requested and is one of several states not included in the study, but officials have said the state’s 2004 recidivism rate was 48.5 percent.

Prince George’s police started by drilling into their data, identifying the neighborhoods plagued by the most murders, non-fatal shootings, robberies and carjackings, Davis said.

They selected five areas — Langley Park, Riverdale, Suitland, Hillcrest Heights and Glassmanor — to focus on, Davis said. Then they looked at the criminals who lived there.

The number was startling, even to police commanders: 233 people convicted in connection with a murder, non-fatal shooting, robbery or carjacking were out on parole or probation, living in or very near the selected neighborhoods. These would be the people called in.

Either by letter or by phone, the Maryland Division of Parole and Probation told the convicts they had to attend one of three sessions or face possible arrest. Adults and juveniles were separated for slightly different presentations.

Less than half actually showed up — though that was largely because more than 100 adults skipped out on one meeting where a probation agent called the convicts rather than sending them certified letters, police said. Those who did not come might face arrest or other sanctions, depending on their individual situations, said Mark Vernarelli, a spokesman for the Maryland Department of Public Safety and Correctional Services.

During one call-in at Parkdale High School, authorities asked the 15 criminals for their help in calling to report crime. Prosecutors warned them “we mean business,” vaguely threatening them with federal charges, mandatory minimum sentences and probation violations should they be charged again.

Afterward, some people complained they were unfairly targeted.

“What is this going to help me do?” said a 28-year-old Riverdale man who declined to give his name, but said he was on probation for burglary and assault. “I feel singled out.”

Until Labor Day, police will assign drug detectives, gang detectives and others to work in the five neighborhoods, and prosecutors will monitor arrests there, especially of people violating their parole and probation, authorities said. Police, prosecutors and other county, state and federal agencies will meet weekly to review the results and tweak the strategy, authorities said.

“We see this as a crime fighting tool, but also as a way to reduce recidivism,” Prince George’s State’s Attorney Angela Alsobrooks said. “We prefer not to see them back in the system.”

The presentation seemed to ring true for those at the call-ins. Even though nearly all of them said they already had learned their lesson from getting in trouble, they also said they were sympathetic to police’s summer effort.

“They just want to stop the crimes,” said Alton Carter, 19, on probation for theft and assault from what started as a robbery charge. “They don’t want no trouble in the streets.”