The police have identified a man named Sirmone McCaulla as a suspect in the stabbing death of Christopher Gutierrez on Sunday in front of the James A. Farley Post Office at Eighth Avenue and West 33rd Street.

On Tuesday, Police Commissioner Raymond W. Kelly held up a picture of Mr. McCaulla and said a police officer and several callers to the city’s tips hot line helped identify Mr. McCaulla, 28, as the man who they suspect stabbed Mr. Gutierrez, 20, based on a series of photographs that were taken by a tourist as he walked away from the scene and that were distributed by the police.

Mr. Kelly said the police were looking for Mr. McCaulla and provided further details of the encounter, after investigators studied police surveillance video. It shows a “fairly crowded” street as the man identified as Mr. McCaulla walks north on Eighth Avenue and Mr. Gutierrez walks south.

They bump shoulders, Mr. Kelly said.

Then there is a short distance, about 10 to 12 feet, during which the deadly encounter might never have happened. But after the men pass each other and keep walking in opposite directions, both turn, Mr. Kelly said. They exchange words. They come back together. It begins to look as if a fistfight is imminent. Mr. Gutierrez takes his coat off and assumes what Mr. Kelly calls a fight stance.

The man identified as Mr. McCaulla does the same, but he appears to be holding something close to his side. It turns out to be a knife. As they get closer, the attacker “lunges, four quick strokes or jabs,” Mr. Kelly said.

Mr. Gutierrez falls to the ground and lies on the steps of the post office. At that point, the man identified as Mr. McCaulla walks away, but then appears to have forgotten something, Mr. Kelly said. He returns to the location “at a relatively slow pace” and retrieves what the police believe is his cellphone, before proceeding north, continuing in the same direction he had been taking before the fatal meeting just moments before.

Mr. Kelly said that Mr. McCaulla had an arrest record, on charges including burglary, and that he had served in the United States Army in Kuwait. It is not clear where he lived.

“We are looking for Mr. McCaulla now,” he told a news conference at Police Headquarters.