A gunman is believed to be dead after shooting one New York Police Department officer and at least two civilians near a corporate office building in New York’s Manhattan.

A man with a long rifle was seen walking into the building, according to one source.

One civilian shot is in critical condition, according to a law enforcement official. Another civilian and the NYPD officer, who was shot in the back, are both expected to survive. The NYPD bomb squad is on the scene.

The officer shot is from a Bronx precinct and was on a post near the building when the shooting unfolded, according to the official.

“Please take proper safety precautions if you are in vicinity and do not go outside if you are near Park Avenue and East 51st Street,” he wrote.

Law enforcement officers at the scene are being cautious and taking cover, a CNN field team on the ground saw.

Video shows heavy law enforcement presence around the building. The police department told CNN the scene is still “very active.”

The 634-foot (193m) skyscraper, located at 345 Park Avenue, includes corporate offices for the National Football League and Blackstone. The building occupies an entire city block and is one of 41 New York City buildings with its own zip code.

CNN has reached out to the NYPD for more information.