Virginia State Police on Friday were trying to determine what exactly drove a gunman with a giant stash of ammunition to shoot and kill a trooper at a Greyhound bus station in Richmond, as the Joint Terrorism Task Force joined the investigation into the shooter's past.

The ex-con, identified as 34-year-old James Brown III of Aurora, Ill., had carried 143 additional rounds of unspent ammunition, Virginia State Police Superintendent Steven Flaherty told reporters Friday afternoon.

"He had a lot of anger about the police in the past," Edith Brown told Richmond TV station WTVR of her nephew, who was shot and killed by state police. "He said he would never go back to prison again... he would fight it out with them."

“Like so many brave Virginia men and women, Trooper Dermyer put on a uniform and risked his life every day to keep us safe, first as a U.S. Marine and then as a police officer.” — Virginia Gov. Terry McAuliffe

Brown previously had been charged with crimes ranging from cocaine possession to murder, serving prison time for domestic battery and aggravated battery of a pregnant woman. Police say he was on his way from Raleigh-Durham, N.C. to Aurora when his bus made a scheduled stop in Richmond.

Trooper Chad Dermyer, a 37-year-old decorated Marine vet and married father of two, was one of a dozen Virginia State Police troopers taking part in a training exercise at the bus terminal when his confrontation with the gunman turned violent.

Flaherty did not reveal what led to the confrontation, but suggested it may have been casual. "I would highly suggest that there was some small talk taking place." Police said earlier that it was part of the training.

The police superintendent said it was still too early to determine whether the shooter had plotted any kind of violent action against law enforcement, but Brown's aunt said there were signs her nephew had a deep-seated hatred for police.

"He pretty much thought he wanted to be infamous... in terms of having a showdown,” Edith Brown told WTVR. “He always praised those people who got into shootouts with police."

Brown pulled out a handgun and shot Dermyer, who was wearing fatigues and no protective vest, multiple times. Two state troopers who were nearby returned fire, and the gunman ran into a restaurant inside the terminal, police said.

Even after police subdued the shooter, and as EMS workers aided him, he continued to be combative, police said. He died later at VCU Medical Center. His gun was recovered, police said.

Details about the training exercise were scant. Police called it “specialized training on criminal interdiction practices,” and said the troopers had completed the classroom instruction and were conducting field practicals at the time the shooting unfolded.

Dermyer, who also died later Thursday at VCU Medical Center, was originally from Jackson, Mich., and had graduated from the Virginia State Police Academy in 2014. He had recently transferred to the state police CounterTerrorism and Criminal Interdiction Unit.

Prior to becoming a trooper, Dermyer had been a police officer in Newport News, Va., and in his hometown. He also served in the Marines for four years.

State and national law enforcement advocates said they are increasingly concerned with an anti-police climate arising from a series of high-profile, racially-charged incidents that they say is getting police officers killed.

“Officers feel like they are targeted and they are being singled out for murder,” Bill Johnson, executive director of the National Association of Police Organizations, told FoxNews.com. “There is a climate out there that increasingly says it is open season on police.”

The Officer Down Memorial Page, which tabulates line-of-duty deaths for law enforcement officers throughout the nation, has overall deaths down slightly year-to-date, but the number killed by gunfire – 15 – is up 150 percent through the first quarter, and on a pace for 60 for the year. In all of 2015, 39 police officers were killed by gunfire, according to the website.

In one particularly deadly week for police around the nation, five police officers were shot to death:

Mesa County, Colo., Sheriff's Deputy Derek Geer was gunned down responding to reports of a person walking with a gun. A 17-year-old suspect has been charged with his murder.

Two Hartford County, Md., sheriff’s deputies, Patrick Dailey, and Mark Logsdon, were killed in a shootout at a Panera Bread in Abingdon, by a wanted man they sought to detain.

Fargo, N.D., Police Officer Jason Moszer, 33, a six-year police veteran, was also killed while responding to a domestic dispute.

Riverdale, Ga., Police Officer Greg Barney was killed exercising a no-knock search warrant at a suspect’s apartment complex.

Law enforcement advocates in Virginia share Johnson’s concern that anger at police could be making the job of protecting citizens more dangerous than ever, said Dana Schrad, executive director of the Virginia Association of Chiefs of Police.

“We are very much concerned about the image of law enforcement in this climate,” said Schrad, whose offices are in Richmond. “To be turned on by the public in this kind of hostile fashion, where Trooper Dermyer was apparently targeted by this individual because he was an officer of the law, is very disturbing.”

Prior to becoming a trooper, Dermyer had been a police officer in Newport News, Va., and in his hometown. He also served in the Marines for four years. Virginia Gov. Terry McAuliffe praised Dermyer for a career spent serving and protecting Americans.

“Like so many brave Virginia men and women, Trooper Dermyer put on a uniform and risked his life every day to keep us safe, first as a U.S. Marine and then as a police officer,” McAuliffe said. “This is a loss that impacts us all. It should inspire prayers for the family, friends and fellow troopers who are mourning tonight, and gratitude for those who protect and serve.“