Police have charged a man they say stabbed and critically injured a customer in a South Philadelphia Starbucks Coffee store on Saturday, but investigators are not sure what led to the attack.

Michael Singley, 52, of the 2000 block of South Juniper Street, was charged with attempted murder and related offenses, police said yesterday.

Detectives still do not know whether there was an argument, as first reported to police, between Singley and the 29-year-old customer, whom police would not identify, Lt. Frank Vanore of the Public Affairs Unit said.

The customer "was just sitting there waiting for his order and was very unclear" why he was stabbed, Vanore said, adding that the victim's condition allowed investigators only a limited interview.

The attack may have been unprovoked, police said. Witnesses reported that there had been an argument, but it may have been more of a confrontation by the assailant, who lunged at the customer, police said.

Singley is accused of repeatedly stabbing the customer about the body and at least once in the face around 5:45 p.m. at the coffee shop at 2201 S. Broad St., between Jackson and Wolf Streets.

Yesterday, the victim remained in critical but stable condition at the Hospital of the University of Pennsylvania, Vanore said.

In addition to attempted murder, Singley was charged with aggravated assault, simple assault, possession of an instrument of crime, and recklessly endangering another person.

Officers arrested Singley about 8 a.m. Sunday at the same Starbucks store after witnesses called police and identified him as the assailant.