Terror at Blackstone: The Harrowing Hours at 345 Park Avenue

The alarm reached Jon Gray on the 44th floor, in his C-suite office high above Park Avenue: An active shooter was in the building.

In the lobby below, a gunman had opened fire, killing a uniformed police officer and two others who worked in the office tower. Then, the shooter fired at the turnstile, boarded an elevator and headed up.

Gray, president of Blackstone Inc., saw his phone light up again and again and again, as security alerts flashed and he fielded calls from associates trying to process what was happening over the next few hours.

Employees hid in bathroom stalls ...