The Washington Post:

The shooter was a political opponent of the councilman, and had accompanied him into the building, a police source said, speaking on condition of anonymity. The gunman's ties to the councilman apparently allowed him to bypass security, the source told The Associated Press.

...

Brooklyn Councilman James Davis, a former police officer and outspoken presence on the council, died after being shot twice in the chest, a city official told the AP, also speaking on condition of anonymity.

The gunman, Othniel Askew, 31, died a short time later at a hospital, according to another police source. The two mortally wounded men were found lying side by side in the balcony overlooking the chambers.

Councilman Charles Barron said he encountered Davis and Askew outside City Hall on their way into the meeting. Davis introduced Askew, telling Barron, "This is the guy who was once against me, but now he's with me." Askew offered a firm handshake and an intense stare, Barron said.

A short time later, Barron stood staring into the balcony as the gunman shot down at Davis' prone body.