A man died while being treated by first responders late Tuesday night after he collapsed outside an apartment building in Westchester, authorities said.

Witnesses said they saw the unidentified man standing near an SUV in an alley behind the gated complex in the 7000 block of Flight Avenue, just east of the 405 Freeway (map), about 11:30 p.m., according to Sgt. Scott Moffitt of the Los Angeles Police Department.

Shortly afterward, the witnesses saw the man lying on the ground near the vehicle, Moffitt said.

Paramedics were called to the scene, and arrived to find the man in full cardiac arrest, police said. He died inside the ambulance as the paramedics were rendering aid to him.

It was unknown if the man had been attacked, Moffitt said.

Detectives responded to the scene and spent several hours collecting evidence and interviewing local residents.

No other information was immediately available.