CHELSEA — The gunshots came without warning, followed by screams.

About 50 people had gathered for a party in an apartment on Washington Avenue when, prosecutors allege, 16-year-old Emanuel Marrero started to fire a gun around 2:40 a.m. Sunday.

When the shooting subsided, Pablo Villeda, 19, had fallen to the ground with mortal injuries, and the alleged shooter’s stepbrother had been struck by gunfire three times, according to a police report.

“People were bleeding. Pablo was laying down on the floor,’’ said Evelyn Martinez, 19, who attended the party with Villeda. “We heard the gunshots. People just started pushing us away, running away for their lives.’’

Martinez spoke Tuesday afternoon after Marrero, a Lynn resident, was arraigned on charges that he murdered Villeda and tried to kill his stepbrother, Nelson Beauchamp Jr.

Marrero stood out of view of the courtroom at Chelsea District Court, where many of Villeda’s relatives and friends had gathered for the arraignment.

Five other people suffered gunshot wounds during the shooting and another party guest was hurt while trying to escape the three-room apartment, according to prosecutors. The survivors ranged in age from 15 to 22.

“The evidence today supports that there was one gun and there was one shooter,’’ said Suffolk Assistant District Attorney Julie Higgins. “And it was this defendant who used that gun, fatally shooting Mr. Villeda as well as others.’’

Though Marrero is a juvenile, state law allows defendants ages 14 or older to be tried as adults for murder. Marrero is also charged with armed assault to murder and assault and battery with a dangerous weapon for the alleged attack on Beauchamp.

Not-guilty pleas were entered on Marrero’s behalf.

Beauchamp, 18, walked on crutches and had a bandage on his chin at Marrero’s arraignment. Another woman who used crutches also attended the proceeding. Both declined to comment, as did Marrero’s relatives.

Higgins said more charges are likely to be filed. Judge Emily Karstetter ordered Marrero held without bail and also revoked his bail on additional juvenile cases he faces in Cambridge and Chelsea. Higgins did not disclose the details of the juvenile prosecutions.

Defense attorney Jonathan Shapiro did not challenge the prosecution’s request to hold Marrero without bail, but he requested that his client be permitted to make a bail argument in the future. He did not return telephone messages Tuesday seeking comment.

Marrero returns to court April 8 for another hearing.

Suffolk District Attorney Daniel F. Conley praised civilian witnesses who provided information that helped put together their case.

“One young man is dead. Six others, some as young as 15, were nearly killed,’’ Conley said in a statement. “The progress we’ve made so far reflects valuable assistance on the street and in the neighborhood.’’

Conley’s spokesman, Jake Wark, said the motive for the shooting is under investigation. He declined to say whether investigators have the firearm that was used.

At her home in Chelsea, Villeda’s mother, Maria Estrada, asked for privacy.

“The only thing I know is that I’m sad and we’re going through a terrible time,’’ she said in Spanish.

Martinez said Villeda attended the party with her, Isabella McLellan, 18, and another friend.

Martinez and McLellan said they were at the gathering for a short time when shots rang out in the living room.

McLellan said she and Martinez ran to an adjacent room, and later found Villeda on the living room floor. He was taken to Whidden Memorial Hospital in Everett, where he was pronounced dead soon after.

The women said they do not know Marrero.

“It was pretty dark in there. You were not able to see anything,’’ Martinez said. “You just heard people screaming, pushing you away, and then asking for help.’’

Even after they exited the apartment, McLellan said, they encountered another man who was wounded.

“We saw a guy shot on the steps,’’ McLellan said. “I tried to help by tying my sweater around his leg.’’

Outside court, Villeda’s friends hugged and reminisced. They said Villeda belonged to a group of friends dubbed, “The Squad.’’

“My heart is shattered from this,’’ said one friend, Ivan Reyes, 21. “He was like my brother.’’

Maria Cramer of the Globe staff contributed to this report. Laura Crimaldi can be reached at laura.crimaldi@globe.com. Follow her on Twitter @lauracrimaldi.