Prince George's County police tweeted today that they now have a third brother in custody in last 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."
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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 PM in Landover, a suburb northeast of downtown Washington, D.C., the chief said.
"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.
"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.'"
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 last 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.