Authorities have said Walker first planted 28 grams of cocaine in the dealer's car and then stole a key to the man's house.
He was booted from the force by Police Commissioner Charles H. Ramsey, but has yet to stand trial on federal charges that include robbery and extortion.
Walker, who worked for 14 years as an officer in the Narcotics Field Unit-South, "was apparently known for being corrupt for quite a while," said defense attorney Bradley Bridge, of the Defender Association of Philadelphia.
Bridge said additional drug cases could be dismissed next month, as prosecutors and the Defender Association continue to evaluate Walker's involvement in each case.
"The process of resolving and reopening the cases is a complicated and messy procedure, but it's important to take care of it and to do it right," he said.
Tasha Jamerson, a spokeswoman for the D.A.'s office, did not respond to a request for comment.
Bridge said his office was contacted yesterday by some of the 12 defendants who had their cases dismissed.
"There's a lot of excitement and family enthusiasm," he said. "We heard from people who hadn't seen their families for years."
Some of the defendants were already out on probation or parole, Bridge said. Others were still behind bars because of drug arrests that involved Walker.
The burly ex-cop was the subject of 18 Internal Affairs complaints that were made by civilians during his 24-year police career.
The complaints - none of which was sustained - included accusations of theft, physical and verbal abuse, and illegal searches.
He was also named in at least eight federal lawsuits, some of which the city settled for tens of thousands of dollars.
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