CHICAGO (CBS) — Three women have been attacked at Northwestern University in the past week, prompting a campus safety alert. At least two of the incidents happened near sororities on the Evanston campus.

Around 5:30 p.m. Thursday, a woman was walking on the 1800 block of Hinman Avenue, when two men approached her from behind. One of the men grabbed the handle of her backpack.

The woman broke free and ran away. Police were notified of the attack, and issued a campuswide alert late Thursday night.

Police said, four days earlier, a student was walking in the 600 block of University Place, on the Panhellenic Sorority Quad, when a man grabbed her from behind and asked for her phone number. The victim was able to get away.

A 21-year-old man with no affiliation to the university was arrested, and charged with battery.

One week before that, another student was walking near Chicago Avenue and Sheridan Road, near the Sorority Quad, when she was grabbed from behind by a man with a heavy Russian accent. He said he wanted to take nude pictures of her.

She ran away and called police.

No one has been arrested in connection with that case.