BOSTON -- Boston police released a sketch Tuesday of man suspected of sexually assaulting a woman walking on Foster Street in Boston's North End on July 4.

The victim said she was approached from behind at about 3:30 a.m., knocked down and sexually assaulted.

The victim fought off her attacker, police said.

The suspect is described as a white male in his 20s. He was wearing a light-colored shirt.

On June 11, a woman reported a sexual assault on Richmond Street near Fulton Street at about the same time of the morning. She also describer her attacker as a white male in his 20s.

There was a string of attacks reported in January. Witnesses gave similar descriptions of the suspect.

Police have not said whether the assaults are related.

Anyone with information is asked to contact Boston police at 1-800-494-TIPS or text the word 'TIP' to CRIME (27463).