WEST TOWN — A woman who was preparing to go inside her home through the front yard was watched by a man on a bike who later climbed the fence into the yard and then sexually assaulted and robbed her, police said.

The assault happened at 11:22 p.m. last Tuesday in the 2000 block of West Ferdinand Street in West Town, according to a community alert sent on Saturday night by Area Central Detectives. The alert said the man was possibly riding a Divvy bike.

According to the alert, a woman preparing to enter her home noticed a man riding a bike in circles in the street. Several minutes after she went inside her home, locking the gate behind her, she told police she went outside and saw the man in her yard.

According to police, the man had apparently climbed the fence to get inside the victim’s front yard. In the yard, the man took out a gun, demanded cash, and sexually assaulted the woman before fleeing the scene, police said.

The man was described as black, 17-25 years old, clean shaven with a dark complexion and wearing his short hair in twists or braids, police said. He wore a white T-shirt, jeans and dark gray hoodie and was carrying a dark backpack.

Police said the man, armed with a black semi-automatic handgun, was riding a blue bike (possibly a Divvy bike).

No one is in custody for the incident, said Officer Patrick McGinnis, a Chicago Police spokesman. A Divvy spokeswoman did not immediately return a request for comment. Last month, armed robbers and carjackers who were riding blue Divvy bikes were wanted by police in West Town and West Loop.

Police advise residents to be aware of their surroundings and to call 911 to report any suspicious persons, vehicles or activity in their neighborhoods. Anyone with information should call Area Central Detectives at 312-747-8380 and reference cases JB-393992, 393992.