What is Rudin Management? Gunman's intended destination not yet clear

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NEW YORK - Four people, including an NYPD officer, were killed Monday when a gunman walked through a Midtown office building and repeatedly opened fire before turning the gun on himself.

It's not immediately clear why Shane Tamura chose 345 Park Avenue as his destination after driving across the country with an AR-15-style rifle. Police said Monday night that the 27-year-old had a history of mental illness.

The backstory:

Police say Tamura walked into the lobby of 345 Park Avenue around 6:30 p.m. and immediately targeted Didarul Islam, an off-duty NYPD officer working as a security guard. He shot more people there, then headed for the elevator where he allowed a woman to walk by unharmed.

Tamura then took the elevator to the 33rd floor and continued shooting, killing one more person before fatally shooting himself. He was reportedly carrying a note in which he blamed football-related head trauma for his declining mental health.

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What we know:

345 Park Avenue - better known to some as the Blackstone Building - is a 44-story tower home to several businesses, including financial firms like The Blackstone Group, Deutsche Bank, KPMG, and Capital Trust.

The global headquarters for the National Football League occupies the building's fifth floor.

The 33rd floor houses offices for the building's owner, Rudin Management Company.

What we don't know:

Investigators have not said why Tamura may have targeted that building. It's not yet clear if he was firing randomly or targeting specific people, or even if he meant to wind up on the 33rd floor.

Dig deeper:

Rudin Management Company is a large real estate firm that owns the building at 345 Park Avenue, along with many other high-end commercial and residential properties across New York City.

Their residential properties mostly include desirable neighborhoods in the Upper East and Upper West sides, while Dock 72 in the Brooklyn Navy Yard is one of their more recent high-profile projects.

The company was founded in 1925 by Samuel Rudin and remains in the Rudin family. It is described by the New York Times as one of the oldest real estate firms in the city; the paper listed the company's holdings at over $2 billion back in 2001.