Into the unknown.

That, said Police Officer Lawrence Leissner, was where he and two other cops were headed Monday night when they responded to a call of shots fired at Building 79 at the Navy Yard.

They did not know they had arrived at the scene of a bloodbath. They did not know how many gunmen - if any at all - were inside. All they knew, Leissner said, was that it was their job to find out.

Yesterday, Leissner, who traded gunfire with and apparently wounded gunman Vincent J. Dortch, recounted the final minutes of a massacre. He had one word to describe his personal feelings: fortunate.

Leissner, 35, also a combat weapons instructor for the Air Force Reserve with more than 17 years in the military, was on solo patrol in South Philadelphia when the call came in around 8:30 p.m. reporting "person with a gun, shots fired, and people shot."

He knew the lay of the land at the Navy Base and arrived with the other officers at Building 79. Together they went inside to the second floor - a high-ceilinged, open office where the lights were turned down low.

"It was eerily silent. There were no sounds, no smells that I could discern immediately," said Leissner, father of a 4-year-old son and whose wife is expecting their second child.

The 6-foot-1 officer said he felt exposed - "like an elephant on the wide-open plains" - in the large, open space.

Walking "point" with his pistol in his hand and with the other two officers behind him, Leissner said he suddenly saw a door open about 25 feet away, a man emerge and, without saying a word, point a gun and fire.

Because the shooting that followed is the subject of a routine departmental investigation, Leissner did not say what happened next, but police brass have said the six-year veteran of the force fired back, apparently hitting Dortch as he ducked behind the door. Dortch then shot himself in the head, police said.

Leissner, who had been awarded a departmental commendation for valor during a deadly gun battle three years ago, said he believed his training both in the military and in the police had paid off.

"Things can go bad for you as you round the corner or open a door," he said. "You never know literally what lies behind the next curve . . . "

"You sometimes have sheer moments or fractions of a second to ascertain what is going on and to take decisive action," Leissner said. "You don't rise to the occasion; you sink to your highest level of training."

The officer said that, when he got home Tuesday morning, his wife asked him if he'd thought of his family at that moment. He had to tell her no.

There were, he said, other unknowns to ponder. Were there other gunmen? If people had been shot, where were they?

He heard someone yelling from behind a wall. It was one of Dortch's co-investors whom he had bound with duct tape and put in an office.

Things soon became clear with the arrival of other officers, and the grisly scene unfolded inside a conference room where three men had been killed and fourth wounded.

Deputy of Commissioner Richard Ross has said Leissner's action perhaps prevented greater bloodshed and allowed help to get to the wounded man, Patrick Sweeney.

Leissner rejects any attempt to label him a hero.

That title, he said, belongs to Sweeney, who - although bound and wounded - spliced together a telephone line Dortch had ripped out of the wall and called 911.

"He had the will to survive," he said. "This is the job I took. I'm not the hero."

Contact staff writer Joseph Gambardello at 215-864-2153 or jgambardello@phillynews.com.