By Joe Holden

NORTHAMPTON TOWNSHIP, Pa. (CBS) — A local attorney shot and killed right outside of his practice. More than a decade later, the murder of Eric Birnbaum remains unsolved.

Feb. 11, 2009. It was an unusually warm day for the dead of winter. Civil attorney Eric Birnbaum just arrived for work. It was 9 a.m.

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“A male subject came up behind him, shot him one time in the back of the head, and the suspect fled the area,” Northampton Township Police Lt. Charles Pinkerton said.

The deadly shooting happened in the law firm parking lot in the Holland section of Northampton Township Bucks County.

Birnbaum’s longtime best friend and law partner Terry Goldberg remembers the phone ringing at home. The shrill screams were hard to understand.

“My first thought was, ‘Oh my God, I can’t understand a word she was saying, but it sounds like somebody died,'” Goldberg said. “it was that kind of screaming.”

A secretary arriving for work at the same time was able to provide a description of the shooter to police, but investigators say it was too vague and they never did catch a break.

“In the years since 2009, we’ve conducted hundreds of interviews in an attempt to establish a motive,” Pinkerton said.

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Who would want Birnbaum dead? His history gave no clues at all.

“I think what is frustrating the investigation is Eric’s squeaky clean background,” Goldberg said. “Like there’s nobody they found yet who would say a bad word about him. Nobody who had an axe to grind with him.”

Police even worked an organized crime angle to no avail.

Goldberg said he spent many sleepless nights trying to unlock the mystery of what happened, to attempt to identify a reason why.

“I was logging onto our computer system as Eric, going through every correspondence, going through every email, going through every file, trying to see or find something that would lead me to a disgruntled client or an upset defendant in a case,” Goldberg said.

Birnbaum left behind two daughters.

“It’s a shame to lose such a close friend and such a good person, great father,” Goldberg said.

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If you have information that may be helpful to police, call the Northampton Township Police Department at 215-322-6111.