Firefighters' trucks and police vehicles are seen as police respond to a shooting incident in New York on Monday.

A gunman is dead after killing one New York Police Department officer and shooting at least two civilians in midtown Manhattan Monday evening, multiple law enforcement sources told CNN.

The NYPD said the shooter acted alone.

Here’s what we know:

Police believe a man with a long rifle walked into a corporate office building in Midtown on Monday, a law enforcement source told CNN. The 634-foot skyscraper, located at 345 Park Avenue, includes corporate offices for the National Football League and Blackstone.

CNN teams on the ground say that within minutes, hundreds of officers responded to the area.

The suspect was found dead inside the building from what officials believe was a self-inflicted injury, multiple law enforcement officers tell CNN.

The NYPD officer has died, a law enforcement official told CNN. A source previously said the officer was undergoing surgery.

One civilian who was shot is in critical condition, according to a law enforcement official. The other civilian is expected to survive, according to one source.

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

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

Officials respond:

The FBI is responding to “provide support” at the “active crime scene,” according to an X post from Deputy Director Dan Bongino.

New York City Mayor Eric Adams confirmed “there is an active shooter investigation taking place in Midtown right now” in a post on X. “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.

Gov. Kathy Hochul said in a post on X she had been briefed on the shooting and urged people to avoid the area.