PHILADELPHIA – Third-year medical student Lea Sullivan walked out of a grocery store Sunday afternoon onto a busy sidewalk and met a savage, inexplicable end.

A burly man in a ski mask attacked Sullivan from behind, clubbing her in the head with a baseball bat and continuing to beat her after she fell. The attack lasted seconds – a minute, tops.

Although South Street was crowded with shoppers, no one tried to stop the attack.

But witnesses did get a good look at the assailant, who removed his mask. “He took it off so everybody could see him,” said Lt. Michael Morrin of the homicide division.

Bystanders also took down the man’s license plate number, leading police to Nader Ali, 26, a former medical student who had been placed on leave from Jefferson Medical College last year because he had been acting erratically, according to a school spokesman. Ali was arrested Monday at his parents’ home in New Jersey.

Police have not disclosed a motive for the attack and refused to comment on Ali’s mental state or provide any other details, other than to say that Sullivan’s roommates described Ali as just an acquaintance of hers.