A tourist helicopter plunged into the East River on Sunday night, killing all five passengers after they were trapped inside the sinking vessel while the pilot scrambled to safety, police sources said.

The Eurocopter AS350 went down around 88th Street, near Gracie Mansion, at about 7:15 p.m., sources said.

The pilot was able to get out and climb aboard a raft — but five others remained trapped inside the upside-down helicopter because they were strapped into their seats, FDNY Commissioner Daniel Nigro said during a Sunday night press conference.

“There was about a minute where we didn’t see anyone come out,” witness Brianna Jesme, 22, told The Post. “Then after a minute, we saw a man come out holding onto a raft screaming: ‘Help! Help! Ah!’ and then inaudible screams.

“It was horrifying.”

Two people were pronounced dead on the scene, while three others were hospitalized in critical condition. They later died at Bellevue, officials said.

Witness Mary Lee, 66, told The Post she saw the chopper go down from her apartment window and said she “immediately thought someone was in there and not going to live — it’s cold water, it was sinking really fast.

“By the time we got out here, we couldn’t see it. It was under water.”

Video posted to Twitter showed a red chopper splash down in the river and then quickly go belly-up as its rotors violently hacked at the water.

“Mayday . . . Mayday . . . Mayday . . . East River engine failure,” the pilot said over his radio during a distress call.

Within minutes, the helicopter disappeared beneath the waves, witness John Magers, 24, told The Post.

“It looked like it was completely submerged,” he said. “It couldn’t have been any longer than 5 minutes.”

The NYPD deployed two divers, and the pilot was taken to Bellevue Hospital, sources said.

Police sources said the chopper was operated by New Jersey-based Fly Liberty Charter and took off from Wall Street.

Fly Liberty could not immediately be reached for comment.

The FAA said it is investigating.