Callahan, who has been on the force for 15 years, has a record that includes suspensions and a federal lawsuit. He has also been accused of threatening his fellow Chicago police officers more than once.
The most recent suspension of 17 months ended in February 2008. Just three months later the 42-year-old was arrested for battery after allegedly attacking two patrons at a Niles Restaurant.
"They told officers that they were attacked for no reason. The male said he was blindsided by the offender (and) he doesn't know why he was beat up. And the woman says she was struck and knocked off the bar stool," said Sergeant Tom Davis, Niles Police Department.
Both victims -- a 50-year-old woman and a 61-year-old man-- were injured in the attack. They told police they had not met Callahan before and after the attack, he fled on foot. A search dog led Niles police to Callahan, who had passed out while hiding in nearby bushes.
"The report I got he had a minor scratch on arm that apparently he had received while he was hiding in the bushes," said Sgt. Davis.
On Sunday, Callahan told a reporter that the 61-year-old man became violent and slammed the woman's head into a bar. Callahan said he then pushed the man and left the restaurant. Callahan also said he was bit by the Niles police dog.
Niles police said Callahan's allegations are not true.
"At the time of the arrest Mr. Callahan was given the opportunity to speak to officers on his side of the story. He never gave his side of the story," said Sgt. Davis.
The Chicago Police Board is familiar with Callahan.
In 2006, former police superintendent Phil Cline wanted Callahan fired after he was accused of threatening several police officers at the 311 Center in 2004. Instead, the police board suspended Callahan for 17-months. Cline filed a complaint after the decision.
In January 2006, Callahan was suspended for 30 days for being intoxicated while off duty in 2004. In that suspension, a Northwest Side bartender sued Callahan for attacking him after he stopped serving the officer. That lawsuit was settled in the city out of court for $15,000.
Also, in 2004, Callahan received a 16-day suspension for being absent without permission.
Callahan has been relieved of his police powers pending an internal investigation.