A woman reported being attacked from behind by two suspects on Erie Alley near 13th Street on June 15. The suspects placed the victim in a bear hug and sexually assaulted her, police said.
The first suspect is described as a 5-foot-4-inch, 140-pound black man between 20 and 25 years old, with a piercing on his upper right eyebrow. The other suspect is described as 5 feet 10 inches tall and is believed to be Hispanic.
Police have released a sketch of the first suspect.
Investigators are also looking into a sexual assault on June 18 that may be connected to the June 15 attack.
That attack, which happened about five blocks away from the first assault at 19th and Capp streets, was not reported, but investigators located the victim after the story surfaced in the "Mission Local" blog.
That victim, who was robbed after the assault, described her attackers similarly to the suspects from the June 15 case.
Police have increased patrols in the area.
Anyone with information on either case is asked to call the San Francisco police tip line anonymously at (415) 575-4444 or text a tip by sending TIP411 and type "SFPD" followed by your message.