CHEVERLY, Md. (AP) — Three brothers have been arrested in the killing of an officer outside a police station by a gunman who “intended to die during a gun battle with police,” his department announced Monday.

Prince George's County police block the road to the police station, Monday morning, March 14, 2016 in Hyattsville, Md.  Police and the public were seeking answers Monday after a gunman opened fire outside a police station Sunday afternoon in a Maryland suburb of the nation's capital, killing an officer in what the police chief called an "unprovoked attack."  (AP Photo/Jessica Gresko)

Prince George’s County police block the road to the police station, Monday morning, March 14, 2016 in Hyattsville, Md. Police and the public were seeking answers Monday after a gunman opened fire outside a police station Sunday afternoon in a Maryland suburb of the nation’s capital, killing an officer in what the police chief called an “unprovoked attack.” (AP Photo/Jessica Gresko)

Prince George’s County police tweeted Monday that they now have a third brother in custody in Sunday night’s slaying of Officer Jacai Colson.

Colson was pronounced dead at a hospital. The gunman was wounded when other officers responded, and is expected to survive. Police have not released any of the suspects’ identities.

Prince George’s County police chief Hank Stawinski called it an “unprovoked attack.”

He said Colson’s fellow officers were going about their business on the quiet, rainy Sunday when the gunman opened fire on the first officer he saw outside the station around 4:30 p.m. in Landover, a suburb northeast of downtown Washington, D.C., the chief said.

A gun battle followed, with several officers shooting at the suspect, Stawinski said.

This undated photo provided by the Prince George’s County Police Department shows officer Jacai Colson, a 4-year veteran of the Maryland county’s police force. A gunman fired outside a Maryland police station on Sunday, March 13, 2016, prompting a gun battle that killed Colson and wounded the suspect, authorities said. (Prince George’s County Police Department via AP)

“Those officers did not shrink. They bravely advanced and engaged this individual,” the chief said.

An eyewitness said she saw a man dressed in black firing a handgun as she looked through her window, according to a report in The Washington Post (http://wapo.st/1M12KZ1).

“He fired one shot, and then he started pacing back and forth, then fired another shot,” said Lascelles Grant, a nurse. Police then poured out of the station, she said. “Just looking outside, I’m like, ‘Oh my God, look at all these police officers running out, putting their lives really in danger.’”

The woman couldn’t immediately be reached by The Associated Press.

Stawinski said one brother was believed to have been with the gunman when he opened fire, but fled and was later arrested. No information was immediately released about the second brother. The chief had few other answers for reporters at a Sunday night news conference.

Prince George’s County State’s Attorney Angela Alsobrooks called it “cowardice” and a “horrific act of evil,” promising an aggressive investigation and prosecution. The FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives also are aiding police, spokesmen for the two federal agencies said.

Colson, who would have turned 29 this week, was a four-year department veteran who worked as an undercover narcotics officer. His football coach at Randolph-Macon College, where Colson played for one year, said he was “a great young man who was well liked and well respected.”

“He was just a great human being,” coach Pedro Arruza said. “He was a very positive, positive person and an upbeat guy, a good person to be around. He had a lot of friends on campus, everybody liked him. He was just a really high-character guy.”

Sheriff’s deputy Dominick Chambers, a friend from the police academy, said they celebrated their four-year anniversary as officers on March 12, the day before Colson was killed.

“He always wanted to be a police officer,” Chambers said. “Everyone is taking it real bad, real bad. I’m talking to my classmates, checking in on them. We’re not doing well.”

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Associated Press Writer Juliet Linderman in Baltimore contributed to this story.