An off-duty Maryland state trooper who had planned to take a youth mentoring group to New York on Friday was ambushed and killed by a gunman early that morning after intervening in a dispute over a bill at a Forestville restaurant, authorities said.

Wesley Brown, 24, was working as a part-time security guard at Applebee's Neighborhood Grill and Bar on Donnell Drive when he was shot about 12:30 a.m. in the parking lot. Covered in blood, he stumbled into the crowded restaurant and collapsed as patrons called 911. He was pronounced dead at a hospital.

The shooting occurred about 30 minutes after Brown had removed a disorderly customer who had refused to pay his tab, authorities said. That customer is considered a "person of interest," Prince George's County police said, adding that they believe he returned and shot Brown. They are reviewing surveillance tape from the restaurant and a nearby bank and have interviewed at least 50 patrons and restaurant workers.

Investigators do not know how an argument over a restaurant check could have escalated into the ambush of a police officer and are trying to make sense of the killing. "It is something that is unfathomable. It shocks the conscience," said Maj. Andy Ellis, a spokesman for the Prince George's County Police Department, which is overseeing the investigation.

Brown was not in uniform, but he was wearing a jacket that displayed the word "police," and had parked his marked police car in front of the restaurant. He was killed just a few feet from the cruiser.

The shooter "knew this was a trooper," Ellis said. Brown was shot while talking on his mobile phone, authorities said. It wasn't clear how many times he had been shot, but police found several casings in the parking lot.

Ellis called the slaying a "cold-blooded killing."

"It doesn't appear [Brown] even had a chance to draw his weapon," Ellis said. "It appears he was ambushed from the outside."

Brown was wearing a bulletproof vest, but the bullets somehow entered in a way that at least one pierced his heart, Ellis said.

Police described the suspect, who fled on foot, as an black male between 5-foot-6 and 5-foot-8, about 130 pounds, and having short hair and a bit of a beard. He was wearing a Hugo Boss jacket. Ellis said the identity of the shooter is "a mystery at this point. We don't have a name or know who this guy is" but is considered armed and dangerous.

The gunman "took a whole lot from this family," said Brown's father, Sylvester, who sat with family outside his Northeast Washington home Friday afternoon. "If my son had arrested him, this wouldn't have happened, but that's not how my son is. He just told him to leave."

Brown, the youngest of nine children, had overcome a youth marked by occasional trouble, family members said, and had recently proposed to his girlfriend, a D.C. police officer. They planned to be married next year.

"He was a remarkable young man. He was on his way to becoming a fine trooper," said Maryland State Police Sgt. Rodney Morris, his supervisor. "All he wanted to do was serve Prince George's County."

Brown was a 3½-year veteran of the state police and an active member of the Seat Pleasant community. He had attended Crossland High School and studied criminal justice at Prince George's Community College in Largo. He was the youngest graduate in his police academy class.

In 2007, he founded Young Men Enlightening Younger Men, a mentoring group that takes teenagers on field trips throughout the Washington area, including to the National Zoo and museums, and brings in professionals to give talks. He was scheduled to take the group to New York the morning he was killed, family members said.

Brown described the group's mission on its website as helping kids who struggle academically.

"What happens to those who try, but who just don't make it because of poor school systems or a lack of support from home? Where does he go?" Brown wrote. "We believe that if a young man is trying to make himself a better man and a productive member of society, then we are PROUD of him."

Sylvester Brown said his son's interest in mentoring stemmed, in part, from a difficult experience he had while growing up. When Wesley Brown was in high school, he stepped in to help a friend who was in a fight and was stabbed, Sylvester Brown said.

Anthony Johnson, 15, a cousin of Brown's who played police- and karate-themed video games with him, said Brown kept a close eye on him and other young family members. One time, Johnson recalled, back when he was about 5, he intended to set fire to a trash can when Brown spotted him.

"He said, 'What are you doing?'" Johnson recalled. "I said, 'I'm doing nothing' and tried to hide the matches behind my back."

His cousin ordered him to hand over the matches, Johnson said, and they went inside for a talk about the dangers of setting a fire.

The last Maryland trooper to die in the line of duty was Mickey C. Lippy, 34, a paramedic who was killed in September 2008 when his medevac helicopter crashed in bad weather in Prince George's.

Brown had finished his most recent state trooper shift early Wednesday and was not scheduled to work again until Monday afternoon, said state police spokesman Gregory Shipley.

Authorities had roped off the crime scene Friday morning, with Brown's police car still parked in front of the restaurant. Dozens of cadets from the state police academy combed the grounds in a grid search for evidence.

"There are a small number of brutal, cold people that would take another's life without thinking about it," Gov. Martin O'Malley said. "That's what happened here."

Brown had wanted to enlist in the Army, his father said, but the family was worried that it was too dangerous, so he decided to become a state trooper instead.

"I'm going to miss that smile," said Theresa Brown, an aunt. "He'd smile all the time, before he even talked."

Maryland flags will be flown at half-staff in honor of Brown.

Brent Jones of the Baltimore Sun contributed to this article.