An autopsy on Jacai Colson showed he was "inadvertently shot by a fellow responding officer during the gunbattle," police said a day after the attack.
They identified the suspect who shot at the building as 22-year-old Michael Ford. He is in custody and remains hospitalized after being injured during the exchange of gunfire. His brothers, Elijah Ford and Malik Ford, are also in custody.
All three will be charged with second-degree murder, six counts of attempted first-degree murder, nine counts of use of a handgun in the commission of a felony and additional charges, according to police.
Police said the shooting was recorded by the Ford brothers.
Michael Ford dictated his last will and testament immediately before the attack, police said. Ford's injuries are not thought to be life-threatening.
In addition to firing at officers and the station, Ford fired at passing cars and at an ambulance, police said.
Colson, a four-year veteran of the Prince George's County Police Department, would have celebrated his 29th birthday this week.
"He took action and in doing so demonstrated extreme heroism because it was his actions that allowed officers to take a position where they could intervene and neutralize the threat," Chief Hank Stawinski said about the fallen officer.
When asked about a possible motive, the chief said the attack was unprovoked.
"This is about nothing," he said.
Colson was assigned to the department's narcotic enforcement division and had worked as an undercover officer -- though that had nothing to do with his killing, Stawinski said.
Fraternal Order of Police President John Teletchea described the attack as unprecedented and unacceptable.
He remembered Colson as a "son, brother, friend, police officer, and a hero" -- someone with an infectious smile and tremendous personality.
"Jacai could light up a room. He lit up everybody's life he touched. He made a decision several years ago to make a difference in the world when he joined the Prince George's County Police Department," Teletchea said. "He made a difference everyday he was here, and he made the difference and saved lives yesterday."
In a statement, Maryland Gov. Larry Hogan said he was shocked and saddened by Colson's killing. He ordered that flags fly at half-staff in his memory.
Attorney General Loretta E. Lynch similarly mourned the officer's death, and called shooting at the station a "heinous act of violence and a cowardly crime."
"His tragic loss is a reminder of the threats that public safety officers face every day, and the dangers that they bravely confront, in every jurisdiction across the country. The Department of Justice will offer any possible aid to the Prince George's County community as they investigate this terrible crime, and we will continue to do all that we can to protect and support our officers and hold wrongdoers accountable," she said.