Detectives hunted for an unknown gunman who shot and killed a Philadelphia-area lawyer in broad daylight in a parking lot near his office.

Police in Bucks County, Pa., were baffled by the brazen crime that took the life of the personal injury attorney, 51-year-old Eric Birnbaum of Philadelphia, on Wednesday.

They searched through the night and were stopping motorists in the area Thursday morning who might have any information about the crime.

Birnbaum was shot at point-blank range in the back of the head about 9 a.m. Wednesday in the parking lot outside his office, Northampton Township Police Chief Barry Pilla said.

He had just parked his black Acura sedan in the strip mall where his firm, Terry D. Goldberg & Associates, was located and was talking to a female coworker, The Philadelphia Inquirer reported. The shooter appeared "from out of nowhere" wearing a knit cap and sunglasses and fired a single shot, according to the woman who was with Birnbaum.

The assailant fled in a minivan.

"We have no idea what could have precipitated it," Pilla told the Inquirer.

The colleague he'd been chatting with dropped to the ground in fear after the shooting, so didn't get a good look at the suspect, the Inquirer reported.

Birnbaum was rushed to St. Mary Medical Center in Langhorne, Pa., where he died a few hours later.

Police haven't arrested a suspect and don't yet have a motive for the shooting. They're asking the public for help.

"We're specifically interested in anyone that may have been traveling north or south on Buck Road in the vicinity of the scene between 8 o'clock, 7 o'clock in the morning until 9, 9:15," Pilla said.

Police stopped someone driving a vehicle similar to the one the gunman fled in and questioned him, but determined later Wednesday that that person was not the gunman.

Birnbaum was divorced with two college-aged daughters. Police say they know of no clients or other lawyers who had any disputes with him, the Inquirer reported.

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The Associated Press contributed to this report.