Live updates: 4 people, including NYPD officer, killed in Midtown Manhattan shooting, official says

Four people, including a New York Police Department officer, are dead after a mass shooting in midtown Manhattan this evening, a law enforcement official told CNN.

The shooter, tentatively identified as 27-year-old from Las Vegas, is dead from what police believe was a self-inflicted injury. The NYPD said he acted alone.

Here is the latest in this fast-developing story:

The victims: An off-duty police officer who was working as a security detail in the Manhattan building was killed after being shot in the back, sources told CNN. Three other people were also killed.

The response: Hundreds of law enforcement officers responded to the scene within minutes, according to a CNN team in the field. In addition to the heavy local presence of police, the FBI is responding to “provide support, according to Deputy Director Dan Bongino.

The shooter: He has been tentatively identified as a 27-year-old from Las Vegas and had an expired private investigator license, sources told CNN. Multiple law enforcement officials also say he had a concealed carry license for a handgun. Police backtracked the suspect to his car, which video shows him exiting alone.

The building: Today’s shooting took place at a skyscraper in a crowded part of Midtown Manhattan, just blocks from Grand Central Station and the Museum of Modern Art.. The 634-foot skyscraper, located at 345 Park Avenue, includes corporate offices for the National Football League and Blackstone.