MELBOURNE, Fla. (AP) — Three teenage girls were joking around and taking pictures on a narrow bridge when they were hit by a train, killing them as a friend watched helplessly, police and a witness said Sunday.

The girls and the fourth teenager, a boy, had been hanging out in Melbourne’s downtown area — known for its shops and nightclubs — when they decided to cross the trestle around 6:30 p.m. Saturday, Lt. Curtis Barger said. Their parents had dropped them off at a mall, and then they took a bus downtown where they were ‘‘just goofing off,’’ he said.

The boy yelled for the girls to run when he saw the train approach, then told them to jump, Barger said. Crane Creek, about 20 feet below the bridge, is slow-moving and about 10 feet deep. The girls did not have enough time.

Bruce Dumas, 53, said he was fishing under the bridge when he saw the teens walk onto the trestle around sunset. He warned them to be careful, but he said they didn’t pay much attention to him.

‘‘You know how kids are,’’ Dumas said. ‘‘They probably wanted pictures of themselves on the track.’’

The girls were about midway across when the train barreled down the tracks, blowing its whistle continuously, he said. Dumas said he could hear the sound of the brakes. After the impact, he heard a girl screaming and crying.

‘‘I think the train was on them so fast they froze and didn’t know what to do,’’ Dumas said. ‘‘It’s crazy to watch a young life snuffed out like that. They didn’t have a chance to live yet.’’

The teens could have jumped onto an old, rusty trestle next to one they were on, though it was unclear why they didn’t.

Barger said all the teens were from the area, but their identities weren’t likely to be released until Monday, after officials can compare dental records.

Ralph Smith said one of the teens killed was his niece, Jennifer Reichert, who was 15 and went to Southwest Middle School in Palm Bay.

He did not want to comment any further.

On Sunday, there was little evidence of the tragedy. A concrete post says, ‘‘Private property, no trespassing.’’ Another sign had fallen onto the ground and was covered in gravel, and a third was twisted and difficult to see.

Graffiti with the words ‘‘more love,’’ with the ‘‘o’’ in the shape of a heart was on the bridge. Andy Ziegler, a member of the Brevard County school board, said teens have painted graffiti on the bridge for the past three decades, but he had never before heard of an accident there.

John Vallee, 54, lives near the trestle and was watching TV when he heard a loud screech. He told the Florida Today newspaper he went outside and first thought he saw a blanket tangled under a rail car. Then he realized it was a person.

‘‘It’s going to be hard for me to get to sleep,’’ Vallee told Florida Today. ‘‘I can’t get it out of my mind.’’

Authorities in Melbourne, a city of about 77,000 nearly 50 miles southeast of Orlando, are investigating.

The track is owned by the Florida East Coast Railway, which operates about 350 miles of track along the state’s east coast. Railway officials did not immediately return calls for comment.