Michael McKeon, a spokesman for the company, said Mr. Camara was supposed to leave work at 4 p.m. on Friday, over an hour before the shooting, but had volunteered to stay late. He said Mr. Camara was armed, trained and “knew that building better than anyone else.”

“It’s clear from the facts that he never had a chance to defend himself in this instance,” Mr. McKeon said in a statement. He said Mr. Camara had worked for FJC Security Services since it took over security operations at 201 Varick Street two years ago.

He said company records did not indicate any link between Mr. Camara and Mr. Downing.

Hector Figueroa, president of Local 32BJ of the Service Employees International Union, which represents security guards in New York, said his organization was “shocked and horrified” by the Mr. Camara’s death.

“Security officers around the city and country serve on the front line each and every day to keep us safe and secure,” Mr. Figueroa said. “We are heartbroken that one of our own has fallen.”

Image Kevin Downing

Mike Charles, a deliveryman who said he had been friends with Mr. Camara for 10 years, described him as a soft-spoken, married father who was quick with a joke.

The two became friends because Mr. Charles delivered packages to the federal building each morning. He said Mr. Camara had worked at there for at least 10 years.