WILMINGTON, Del. — A gunman opened fire this morning in a Delaware courthouse lobby just as the building was opening to the public for the day, exchanging shots with police and leaving three people — including the shooter — dead, authorities said.

“It happened so fast,” said Jose Beltran, 53, an employee at the New Castle County Courthouse who was entering the lobby when he heard two shots. He said he turned around and heard three or more shots as he ran.

Delaware State Police Sgt. Paul Shavack said the suspected gunman and two women are dead. The city’s mayor said one of the women killed was the shooter’s estranged wife, but Shavack said police had not confirmed that was the case.

Shavack did not say how the gunman died. He said two police officers suffered non-life-threatening injuries.

He said the shooting was not random, with the motive still to be determined, and he cautioned against information coming from any other source.

But Wilmington Mayor Dennis Williams said in a telephone interview that he was told the man shot and killed his wife.

Shavack said the gunman opened fire before he passed metal detectors in the lobby.