NEW YORK (AP) — New York City's mayor says three of the four people critically injured in the East Village fire and explosion have been released from the hospital.

Mayor Bill de Blasio said at a news conference Monday that the remaining patient was in Bellevue Hospital and "doing well."

He said fire marshals, the NYPD and the district attorney's office were investigating the cause of Thursday's apparent gas explosion. It killed two people and leveled three buildings.

De Blasio said he hasn't ruled out policy changes, or tougher penalties for those caught tampering with gas, but the investigation must conclude that first.

The mayor says most residents in the surrounding area who were evacuated immediately after the blast have returned to their homes.