A shoplifting suspect was shot and killed Wednesday after he stabbed one plainclothes San Francisco sheriff's deputy and threatened another in a street confrontation in the South of Market, authorities said.

The incident began shortly before noon when two men, being chased by store security guards after a shoplifting incident at a Walgreens on Market, literally ran by Senior Deputy Daniel Rosen and his partner at Ninth and Mission streets.

Rosen, head of a special team looking for fugitives known to hang out in the area, went after the suspects, Sheriff Michael Hennessey said. It is unclear whether the deputies identified themselves.

Rosen tackled one of the fleeing men, who allegedly pulled out a knife and stabbed Rosen in the thigh, the sheriff said.

The man then turned and threatened Rosen's partner with the knife, authorities said. The deputy, identified only by his last name, Cabrera, shot the man, authorities said.

The dead man had no identification, and his name was not immediately known, Hennessey said. The sheriff could not identify the second man, who he said surrendered after the shooting.

Bill Travis witnessed the incident while eating at a restaurant on Ninth Street.

"I heard a scuffle outside the window," Travis said. "One man said, 'Drop the knife!' Just before I got to the door, a shot was fired.''

He said he knew the people were involved were law enforcement because of the radio microphones clipped to their collars, but he did not hear them identify themselves as deputies.

Travis said the man was shot in the abdomen and treated on the scene. "They cuffed him and before the paramedics got there, the cops were pounding on his chest and checking his pulse and all,'' he said. The man who was shot was pronounced dead at San Francisco General Hospital.

He said a backpack was dropped nearby and a bottle of shampoo -- apparently taken from the store -- had fallen out.

Rosen was also taken to San Francisco General, where Hennessey visited him and said he would be able to return to work soon.

Both men are veteran deputies, Hennessey said, and their team makes hundreds of arrests each year without incident.

The last time a deputy shot and killed a person in the line of duty, several years ago, happened to involve a man wanted on an outstanding warrant who attacked deputies with a knife, the sheriff said.

Despite the rarity of those violent encounters, Hennessey said his deputies are prepared to deal with them.

"Deputy sheriffs are trained to do the right thing, and in this case I believe they did," Hennessey said.

"Nobody wants to see anyone get killed in this kind of situation, not even the bad guys," he said. "But if you attack a peace officer with a deadly weapon, you're likely to get shot, and if you get shot you're likely to die."