(CNN) The gunman who shot and injured a paramedic in Dallas before apparently killing himself was the subject of a federal investigation prior to Monday's shooting, the FBI said Tuesday.
Special agent in charge Eric Jackson of the FBI's Dallas Division would not elaborate, saying only that Derick Lamont Brown was well-known to local, state and federal law enforcement officials.
Brown had claimed on his Facebook page to be the New Black Panther Party's longtime national minister of defense. The group did not respond to CNN's email seeking comment on Brown's relationship to the organization.
He "had been acting erratically in his home yesterday morning" and at some point took his roommate, Arthur Riggings, 67, into a back room and fatally shot him, Dallas Assistant Police Chief Randall Blankenbaker said.
The paramedic -- identified as William An, a 10-year veteran of the department stationed in east-central Dallas -- remained in intensive care Tuesday.
The paramedics apparently believed they were responding to a suicide call, Dallas Fire-Rescue Department Chief David Coatney told reporters.
"They served to try to take care of a citizen," the chief said. "Unfortunately, we had a tragic event where they were shot at."
Brown had numerous charges on his record, mostly involving drugs, weapons or driving while intoxicated, as well as several misdemeanors, and in most of the cases he took plea agreements, court records show. He had been arrested on allegations of aggravated assault with a deadly weapon, police said.
He was sentenced to jail on at least nine occasions, but it is not clear from inmate records how much time he actually served.
Brown went by "Brotha Dk" on Facebook, where he posted occasionally about the New Black Panthers and on at least one occasion in 2014 shared a photo of himself holding a single-barrel shotgun. He had served as the organization's defense minister since 1998, according to the Facebook page.
A law enforcement source familiar with the investigation confirmed the page belonged to Brown.
The Southern Poverty Law Center describes the New Black Panthers as a black separatist group known for "its anti-white and anti-Semitic hatred."
The new group has no affiliation with the original left-wing militant group of the 1960s and 1970s, the SPLC says. That echoes a 2014 statement issued by the Dr. Huey P. Newton Foundation, named for the original Black Panther Party's co-founder, which read, "The people in the New Black Panthers were never members of the Black Panther Party and have no legitimate claim on the Party's name."
After Brown killed his roommate, Blankenbaker said, Brown's girlfriend fled the home. A 24-year-old neighbor who heard gunshots came outside, and Brown shot him.
An and another paramedic, responding to a report of a possible suicide, arrived and began treating the neighbor when Brown opened fire on them with a semiautomatic rifle, Blankenbaker said. An was critically wounded, while the other paramedic was able to escape, the assistant chief said.
Dallas police Officers William Bruce and Thomas McPherson arrived and saw Brown outside with rifle. Brown began shooting, and the officers took cover, Blankenbaker said. Bruce suffered a calf injury, possibly from bullet fragments, he said.
An officer fired a shotgun at Brown, and a pellet may have hit Brown, who retreated into his home, Blankenbaker said.
With Brown still firing at police, Officers Matthew Merta and Matthew Kalash, along with Sgt. Robert Watson, secured An and put him in Watson's car. Watson rushed the injured paramedic to Baylor Hospital. Other officers took the wounded neighbor to safety.
"All of these officers' actions are more than commendable and should be considered heroic," Blankenbaker said.
After Brown took his own life, SWAT officers used a robot to enter the home, where it found Brown's roommate dead.
An underwent surgery and was in critical but stable condition, Dallas Fire-Rescue Department spokesman Jason Evans said. An and the wounded citizen were in the intensive care unit at a hospital.
The paramedic is a 36-year-old married father of one with another child on the way, Evans said. He has undergone several procedures to treat his injuries, Evans said, and though An complained of pain Monday night, his "outlook seems to be a lot more positive."
His wife, Jayne, released a statement saying her husband was OK. She thanked those who directed their prayers her family's way.
"Please continue to pray for Will and keep him and our entire family in your prayers. Special thanks again to the first responders who risk their lives daily for others. God bless," she said.
Bruce, the officer possibly hit by a bullet fragment, was treated at the scene for an unspecified injury, authorities said.
Police closed roads near the intersection of Interstate 30 and Dolphin Road -- east of downtown -- shortly after the shooting.
"Confirming 1 paramedic shot while responding to a call. No detail at this time. Thoughts and prayers are with all impacted," Dallas Fire-Rescue tweeted at that time.
The shooting happened near the Dallas Fire-Rescue's Dodd J. Miller Training Center, about a quarter-mile south of Interstate 30.
The FBI is assisting Dallas police in the investigation, according to FBI spokeswoman Lauren Hagee.