Boston police are searching for a suspect they believe sexually assaulted two women in Brighton on Thursday evening, police said Saturday. Robert Merner, the head of investigative services for Boston police, said both victims described the suspect as a white man in his late 30s or early 40s, 5 foot 8, with a slim build and short brown hair that is balding at the crown of his head. A surveillance photo that evening shows a man believed to be the suspect wearing a dark T-shirt, dark shorts, and long white socks with sneakers. Both victims said the man approached them from behind and told them not to speak or scream, Merner said. The first incident took place outside 39 Brighton Ave. at 7:50 p.m., and the second about 40 minutes later near the intersection of Quint and Brighton avenues. In both cases, the women started screaming and the suspect ran away. The department has increased foot patrols in the area, and is encouraging anyone with information to call police at 617-343-4400.