An off-duty Baltimore police officer shot and killed an unarmed man outside a Mount Vernon nightclub early Saturday. Police said the officer fired his service weapon at least 13 times on a crowded street after witnesses said the victim groped the officer's female companion.

While police said witnesses agreed that the victim had inappropriately touched the woman and fought with the officer, spokesman Anthony Guglielmi said detectives have "not been able to find a concrete motive" as to why the officer used his weapon.

The victim, identified as East Baltimore resident Tyrone Brown, 32, was hit at least six times in the chest and groin, according to the police spokesman. The officer, a 15-year veteran assigned to the Eastern District patrol division, fired his department issued Glock handgun at least 13 times, officials said.

The officer -- who was not immediately identified in keeping with department policy -- has been assigned to desk duty while the department investigates.

Police commanders indicated that Saturday's shooting was troubling. It raises numerous questions, including whether the officer had been drinking and was impaired when he fired his gun, why he did not call for help from the many on-duty officers stationed nearby and whether it was appropriate for him to fire on an unarmed man in a crowd of bystanders.

Guglielmi said the shooting happened about 1:30 a.m. outside the Eden Lounge on East Eager Street in Mount Vernon. Dozens of people were on the street including several uniformed city police officers who are routinely sent to the area on weekends because of past violence outside nearby bar and clubs.

Deputy Police Commissioner Anthony Barksdale has been ordered to be "intimately involved in the investigation." A police spokesman said the officer refused to make a statement and declined to submit to a breath test to determine whether he had been drinking alcohol.

Officers involved in shootings are not required and cannot be compelled to answer questions from homicide detectives conducting a criminal investigation, though most do so after consulting union attorneys. Officers can be ordered to give statements only after the criminal investigation is complete, for the purposes of administrative review.

Guglielmi said the police commissioner was apprised of developments throughout the night and the commissioner ordered "his most trusted people" to ensure a thorough investigation.

Police said the officer was with a group of people inside the Eden Lounge late Friday and early Saturday. They left about 1:30 a.m., joining hundreds of others spilling out of bars and nightclubs in the neighborhood known for late-night revelry.

Guglielmi said Brown "approached and made advances toward the officer's female companion." McLarney said Brown grabbed and groped the woman. Police said the officer argued with the man, and then the altercation became physical.

McLarney said the officer identified himself as a member of law enforcement and gave "verbal commands" to Brown to stop fighting. At some point, Guglielmi said the officer took out his semi-automatic service weapon and fired. Officials said the two men were standing just a few feet apart.

A patrol officer standing in an alley off East Eager Street, used by officers to park their vehicles, heard the gunfire and quickly responded to the scene.

Brown was rushed to Maryland Shock Trauma Center, where he was pronounced dead at 2:17 a.m.

Baltimore police officers are required to carry their service weapons at all times while on and off duty when they're within the city limits. But McLarney said commanders urge them to use common sense and sound judgment. Police are allowed to fire their weapons when they feel their lives or the lives of others are in danger. They are not required to carry weapons when doing so would be impractical, such as playing a pickup game of football or swimming.

There are no rules prohibiting officers from carrying guns into bars, but it is against department regulations to be intoxicated or inebriated while armed. Police officials said if an officer plans on drinking to excess while off duty, common sense dictates leaving their guns secured and at home.

The issue about officers carrying guns at bars has been raised many times in previous years. Police point to several instances in which armed off-duty officers in bars have legitimately used their weapons to defend themselves or others.

In November 2009, a city officer shot a man who stabbed him during a fight outside a strip club, Christina's Female Revue, in Dundalk.

In March 2008, authorities said a city police officer stood outside a Canton bar and fired his service weapon at a car as he sped away, hitting its gas tank. Police said the officer and the driver had gotten into an altercation inside the bar. The officer has a trial pending on a charge of attempted murder.

And in 2007, two city officers patronizing Club Fantasies in South Baltimore's Curtis Bay neighborhood shot and killed a 38-year-old man who tried to hold up the club with a shotgun. The officers, who had just gotten off their midnight shift, fired after the robber fired into the ceiling.

But the practice came under strict scrutiny in 2008 when a city officer shot and killed a colleague, Norman M. Stamp, outside a Baltimore strip club on South Haven Street. Stamp had been out drinking with biker buddies to celebrate his 44th anniversary on the force when a fight broke out and spilled outside.

Three police officers rushed to the bar and confronted Stamp, who was hitting one of the combatants with brass knuckles. An officer Tased Stamp, who stood, pulled out his service weapon, and was shot by an officer twice in the chest. He later died.

Just last month, a block away from the scene of Saturday's shooting, another off-duty police officer shot and wounded a man suspected of breaking into the officers car. That incident occurred at East Chase and North Charles streets, between the Belvedere Hotel and Brewer's Art restaurant and bar.

This latest shooting comes just one day after another city police officer shot and wounded a 26-year-old who police said was burglarizing the officer's home in Dundalk, in Baltimore County. Police in that case said the suspected burglar was shot inside the officer's living room.

City police officers have shot five people this year, killing three of them. Last year at this time, officers had shot eight people. The majority of police involved shootings are ruled justified. But a trial began on Friday in which a city officer, Thomas Sanders, is charged with manslaughter in connection with an on-duty shooting in 2008 in the Hamilton Shopping Center.