Meanwhile, police have found a .40 caliber shell casing that they believe came from the murder weapon.
Birnbaum was getting out of his car Wednesday morning, chatting with a co-worker as he headed into work when someone appeared, seemingly out of nowhere, and shot Birnbaum in the back of the head.
It happened in Gateway Shopping Center in Northampton Township, Bucks County, where the law firm is located.
Thursday morning, police did a rush-hour trafffic stop, questioning commuters and handing out leaflets.
Investigators concede that, so far, they have little to go on.
The law firm Birnbaum worked at remained closed as the hunt for the killer continues.
Birnbaum, a divorced father of two, was a personal injury attorney, and police are scouring the client list to see if someone had a particular grievance with the lawyer.
Longtime friend Marc Kay says, it's hard to imagine who that might be.
"If you interviewed a thousand people right now you would never hear a bad word about Eric Birnbaum," Kay said.
Witensses say the killer was sitting in a car outside a deli at the shopping center before the shooting, and was wearing a pullover knit cap with sunglasses. He's described as a white male, standing about 5'10" tall.
For now, police are not releasing details of murder weapon, only saying it is larger than a 9mm handgun.
Northampton Twp. Police continue asking for the public's help.
If you have any information, call them at (215) 322-6114
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