On his way to a Carnegie Mellon University basketball game, 21-year-old Ben Saks stumbled upon a police officer struggling with a suspect in an alley behind his Shadyside house.

When Mr. Saks, a junior architecture major at Carnegie Mellon, offered his assistance, the police officer accepted. Together, they were able to subdue suspected drug dealer Omar Pagan.

But in the process, Mr. Pagan grabbed hold of the officer's gun and shot Mr. Saks in the hand.

The Saturday night incident left Mr. Saks, who builds architectural models, has participated in model airplane building competitions and said he'd never been in a fight, scared, sleepless and nursing more than 20 stitches.

To experienced law enforcement experts and officers, the incident illustrates the successes and the risks of accepting help from a good Samaritan.

"Police work is generally best left to the police, there's no question about that," said Eugene O'Donnell, a professor of law and police studies at John Jay College of Criminal Justice in New York City. "But there are exceptions."

Mr. O'Donnell, who previously worked as a New York City police officer, effortlessly ticked off reasons why police departments prefer to keep good Samaritans out of the way when it comes to the actual physical process of making an arrest.

For starters, he said, a good Samaritan that comes forward might not actually have good intentions -- the eager volunteer might be on the side of the criminal. Also, he said, even citizens with pure hearts are unlikely to have proper training to diagnose the situation: They might unnecessarily put themselves in harm's way, or even interfere with an arrest to the point where the criminal escapes.

But all of those drawbacks notwithstanding, there are times when an ordinary citizen makes the difference between suspects getting away and getting arrested.

Mr. O'Donnell can remember several of those instances from his years as a police officer.

"I have a high regard for people who righteously step forward," he said. "One of the things we have in big cities is too often people are unwilling to get involved."

Pittsburgh Police Chief Dominic J. Costa thinks that most Pittsburghers would make the same decision that Mr. Saks did in the same situation. But he agrees that while good Samaritans can be invaluable, police should only accept assistance as a last resort.

"If there are other options to assist, obviously there's not a need for a good Samaritan, nor do we need our population to get involved when there's a police officer there and the numbers are in our favor," said Chief Costa.

But in this case, he said, both the officer, who has not been identified, and Mr. Saks did the right thing.

"When the officer has tried everything he has available to him -- you may have someone on the ground and you know your back-up is coming -- I don't care who jumps in to help me if somebody is going for my weapon."

The issue of good Samaritans can be particularly tricky for police departments, because citizens often act without the consent of the officer on the ground.

"Usually it's not left up to the officers at all," said Paul McCauley, a criminology professor at Indiana University of Pennsylvania. "A good Samaritan almost by definition is someone who volunteers, so often there's no dialogue."

Chief Costa asked that good Samaritans try to ask permission before they intervene, as Mr. Saks did.

Chief Costa said that both he and Mayor Bob O'Connor spoke with Mr. Saks to thank him personally.

Mr. Saks, a native of Shaker Heights, Ohio, said except for a $100 co-pay, insurance is covering the cost of his injury and he has not asked the city for any financial help.

"I'm not really asking for anything," he said. "I'm just asking for my health."