Simon was struck several times and was pronounced dead at 11:10 a.m., authorities said. No weapon was used, they said.
Two corrections officers and the exterminator reportedly witnessed the beating.
John Cunningham, vice president of Police Benevolent Association Local 105, which represents corrections officers, said Simon and Harris had been allowed to be together because "there was no prior knowledge they were having a problem."
Cunningham described the recreation area as an 8-foot-by-10-foot chain mesh cage in the middle of the death-row wing, with a table and four stools bolted together for inmates to play chess, checkers or cards.
"They were put in there so an exterminator could fumigate their cells, which is routine," Cunningham said. "The other guys could see it if they were there, but most of them were in the yard. The crime was seen by two officers in the unit, and the exterminator supposedly saw it."
Cunningham said he knew of no motive for the beating.
"Inmates fight each other all the time. I saw an inmate stabbed over a sweatshirt," he said.
Simon, who weighed 210 pounds, and Harris, who weighs 260, had long records of violence. Both were sentenced to death for notorious crimes, and both remained on death row pending appeals.
Simon was sentenced in May 1997 after he pleaded guilty to killing Franklin Township Police Sgt. Ippolito "Lee" Gonzalez, 41. He later tried to recant the plea, saying he had made it to protect a fellow member of his Warlocks motorcycle gang. The New Jersey Supreme Court, however, upheld Simon's death sentence last month.
The murder exposed flaws in the parole systems of Pennsylvania and New Jersey, and the outrage over Simon's parole prompted changes in both states.
Simon was released from a Pennsylvania prison 11 weeks before Gonzalez was killed. He had served part of a 10- to 20-year sentence for killing his 19-year-old girlfriend after she refused to have sex with his Warlocks gang members.
Harris was convicted in February 1996 for the carjacking, rape and murder of Kristin Huggins, 22, of Lower Makefield. Huggins disappeared after driving to downtown Trenton on Dec. 17, 1992, to paint a mural.
Harris has been in trouble with the law since his youth, and most of his adult life has been spent in prison. He was institutionalized at Trenton State Hospital for about a year when he was 12 and diagnosed as "severely mentally ill."
Chief William H. Wright of the Franklin Township police said yesterday that Gonzalez had been "very dedicated and professional, a cop's cop," and he expressed hope that Simon's death might bring some relief to his grieving department.
"There's an irony to the incident, in that the death sentence of Robert Simon was carried out. Unfortunately it was not carried out appropriately," Wright said.
Louis Gonzalez, an older brother of the slain officer, voiced satisfaction.
"I feel real elated the way it happened," said Gonzalez, 49, of Newfield, a section of Franklin Township, where he runs an automotive repair shop. "It was a violent death. He suffered and felt pain. . . . And that's the way my brother died - he died in pain. So I feel that his death today kind of pays off for the pain and suffering that my brother went through."
John L. Call Jr., a lawyer who represented Simon and Harris in their capital-punishment trials, said Simon died the way he lived. "I think he would have preferred to have died this way than to have been strapped to a gurney and have a needle put in his arm," Call said.
"Simon was . . . much more inclined to be aggressive to someone who could fight back than Harris," Call said. "Robert Simon was a very tough, prison-hardened individual, and certainly inclined to violence with the slightest provocation. Even-up, I would have put my money on Simon."
Gloucester County Prosecutor Andrew Yurick, whose office prosecuted Simon, said he would not "lose any sleep over" Simon's murder. "He was a very violent person who had no respect for human life, and he apparently met someone that felt the same way who was a little bigger and a little faster," Yurick said.
He had harsh words for the penal system.
"Mudman Simon was in jail in Pennsylvania for killing his girlfriend. He killed a prisoner. And then he gets paroled and kills a police officer. The criminals know the system, and what they know is there's no teeth to it. Literally, they can get away with murder," Yurick said.
New Jersey has not executed anyone since restoring the death penalty in 1982. The last execution in the state was in 1963.
In February 1995, Simon, a Warlocks member who had spent most his adult life behind bars, was paroled from a Pennsylvania prison after serving 20 years for a string of crimes, including the second-degree murder in 1974 of Beth Smith Dusenberg of Drexel Hill.
With the approval of New Jersey parole officials, he moved to Gloucester County under the condition that he not associate with the Warlocks.
But 11 weeks later, he and another Warlock, Charles E. "Shovel" Staples, shot and killed Gonzalez, who had stopped the two after a burglary alarm.
In the days after the killing, it was learned that New Jersey officials had been lax in monitoring Simon and apparently were unaware that the Warlocks had a presence in Gloucester County. Pennsylvania officials acknowledged that Simon, who had successfully used a self-defense argument in the killing of a fellow inmate in the 1980s, was released amid prison overcrowding even though he had repeatedly been denied parole since 1992.
The Gonzalez murder prompted legislators to pass tougher laws to keep violent offenders in prison.
In Pennsylvania, the parole board reviewed 10,613 applications in 1994, including Simon's. About 72 percent of those who applied for parole were released, said Vicki Wilken of the Pennsylvania Board of Probation and Parole. Two years later, after legislators passed tougher laws, 38 percent were paroled.
"Parole in Pennsylvania is discretionary, and we do it on a case-by-case basis," Wilken said. "If Simon applied for parole today, I'm not so sure he would be released."
After Gonzalez was killed, Pennsylvania conducted a Senate judiciary hearing to review parole cases and policies.
As a result of Simon's release, two parole board members, including the board chairman, were not reappointed and board members were increased from three to nine.
In New Jersey, parole standards changed drastically.
Gov. Whitman created a commission to review and overhaul the parole system. In 1997, she signed legislation designed to overhaul New Jersey's parole system by toughening standards for inmate release and giving crime victims more say in parole decisions.
"It's given us the control to exercise more discretion," said Robert Egles, executive director of the New Jersey State Parole Board. "In the past, if we were inclined to deny parole, we didn't have the power to do so."
Contributing to this article were Inquirer staff writers Joseph A. Gambardello, John Way Jennings and Lacy McCrary.