UPDATED 11/29/10 10:32 a.m.

CHICAGO (CBS) – A man is in custody after allegedly stabbing a sailor during a brawl that erupted inside the McDonald’s across from Wrigley Field early Sunday, police said.

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The incident occurred at 3:45 a.m. inside McDonald’s, 3620 N. Clark St., according to a Belmont District police lieutenant, who said officials were trying to determine who was the aggressor or if the man acted in self-defense.

An apparently homeless man was seated in the restaurant when three sailors approached him and they began “having words,’’ and fighting, according to the lieutenant.

The man stabbed one of the sailors with a steak knife and injured him, sending the 20-year-old man to Advocate Illinois Masonic Medical Center, the lieutenant said. His condition was not known immediately.

There were several witnesses to the incident, and authorities are also scrutinizing video surveillance, according to the lieutenant.

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The 29-year-old man was in custody but had not been charged with anything as of 11:15 a.m. Sunday. No updates were immediately available Monday morning.

This is the second time in recent months that the McDonald’s across from Wrigley Field has made headlines in the police blotter. In September, registered sex offender Cortez Lewis, 29, was arrested and charged with molesting an 8-year-old boy in a restroom there.

Prosecutors say while in the bathroom, Lewis ordered a boy into a stall, and pulled down his pants and the child’s, inappropriately touching the 8-year-old. The child got away, and told his aunt, who chased Lewis for more than a block before Lewis was arrested, according to prosecutors.

Lewis, of the 11300 block of South Wentworth Avenue, is being held on $750,000 bond on charges of felony unlawful restraint and felony failure to register as a sex offender within 10 days. Lewis was also charged with disorderly conduct and battery, which are misdemeanors.

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The Sun-Times Media Wire contributed to this report.