He later died at the hospital.
The shooting happened in broad daylight at the Gateway Shopping Center in Northampton Township, Bucks County.
Police say Birnbaum had just stepped out of his car and was on his way to work at the law firm of Terry D. Goldberg when the gunman walked up to him and shot Birnbaum in the back of the head.
Authorities say Birnbaum had been talking to a co-worker, who was getting out of her car when the shooting happened.
"She does not believe he saw the perpetrator at all. She did," said Northampton Township Police Chief Barry Pilla. "As soon as it occurred, she went down."
An employee at a deli in the shopping center arrived just after the shooting.
"I thought he was having a heart attack," said the man who did not want to be identified. "There were two ladies by him. I though I could, maybe, lift him up. But, when I got out of the car there was blood everywhere and he was shot in the back of the head so I did not touch him."
Police will be back out at the crime scene tomorrow to see if they can find any more witnesses.
Anyone with information is urged to call Northampton Twp. Police at (215) 322-6111.
A "person of interest" apprehended along Bustleton Avenue at Laura Lane after the shooting.
This afternoon, police said that person is not connected to the crime.
Those who knew Eric Birnbaum say he was a great guy.
"So nice, he would help anybody with anything," said longtime friend Mitchell Gerson.
Birnbaum's Facebook page has a quote saying, "Life is short! Work hard, play hard! Be kind, live healthy, enjoy life fully, and learn from your mistakes.
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