HOUSTON — A suburban house party to celebrate a girl’s 18th birthday erupted into gunfire that killed two teens, wounded many others and caused some partygoers to flee for their lives through second-story windows, authorities said Sunday.

Saturday’s party in Cypress, an unincorporated area about 25 miles northwest of Houston, was promoted openly on several social media sites and drew more than 100 people, most of them 17- to 19-year-olds, Harris County Sheriff Adrian Garcia said. The birthday girl’s mother attended the party at her home.

He said people were apparently dancing in the home at about 11 p.m. when someone fired a pistol in the air in celebration. In the ensuing confusion, someone else began firing into the crowd, causing people to stream into the narrow street, Garcia said.

Partygoer Shaniqua Brown — who said she heard about the party through Instagram, a photo-sharing app and website — told The Associated Press it “was not rowdy at all.” She said she first heard gunshots in the house and they continued outside as people fled and sought cover.

After giving varying reports of how many people were injured, authorities clarified Sunday that 20 people were hurt, including 16 who were wounded by gunfire — two critically — and four others who suffered injuries while trying to flee.

The two people killed, one an 18-year-old boy and the other a 16-year-old girl, were students at Cypress Springs High School, Garcia said.

He said investigators were seeking two suspected gunmen — one of whom is believed to be about 17 years old and the other who is thought to be about 22.

Garcia chastised the party organizers, who advertised the event on social media, saying “you have no control on who to expect at your door.”

He said the organizers arranged to have people searched as they entered the home. “Anytime you have to factor in a bouncer and being searched at the door, you have already taken a turn for the worse,” he said.