HOUSTON — Houston police are looking for two suspects after six men were shot and wounded early Sunday at an after-hours nightclub.

Four of the wounded are in critical condition, said Assistant Chief James Skelton during a news conference. He provided no information on the conditions of the two other victims.

Dispatchers were called around 3 a.m. to the packed nightclub, which was still serving drinks after the city’s bars are supposed to be closed, Skelton said.

He said police are reviewing video and talking to witnesses, but the preliminary investigation suggests that the people involved knew each other. He described it as an “isolated attack” that illustrates the problems of after-hours clubs.

“Establishments like this, that harbor within our city, contribute to crime,” Skelton said. “So the Houston Police Department, we are targeting these locations, and we’re going to shut them down because it harbors the DWIs and the assaults, and that’s exactly what we saw here tonight.”

Officers are looking for a black car with the two suspects in it, he added.