She never saw it coming.

The woman who was attacked outside a West Ashley medical office complex Monday was preparing to make a phone call when a stranger slipped in her car and demanded she drive away, Charleston County Sheriff Al Cannon said. They ended up at a vacant house, where the man sexually assaulted her in the back yard, he said.

Investigators released a composite sketch of her attacker Tuesday. They have no suspects, but Cannon said it appears the man was familiar with the office complex and the site where the assault took place.

Cannon said he knows of no similar attacks in the area. The Sheriff's Office is comparing notes with Charleston police and working with other agencies to get a bead on the attacker, he said.

"We are working with all the resources we have to locate this individual and take him into custody," he said.

The incident occurred around 3 p.m. Monday outside West Ashley Primary Care at 2270 Ashley Crossing Drive, deputies said.

The 25-year-old victim, who works for a pharmaceutical company, was in her Ford Escape preparing for a call with her employer when the stranger jumped inside, Cannon said.

"It happened very quickly," he said.

"She was totally unaware of anyone or anything around her when it happened."

The man threatened the woman and forced her to drive off, Cannon said. No weapon was shown. They traveled about a mile to a vacant house that is for sale on Sunnyvale Drive. He forced her into the backyard, which is largely hidden from view by surrounding woods, he said.

The man sexually assaulted the victim, forced her to disrobe and threw her clothes over a fence, Cannon said. The attacker then drove off in the woman's car, an action witnessed by a man in the area, he said.

The woman, meanwhile, gathered her clothes and then ran down the street yelling for help, Cannon said. A concerned citizen spotted her and called 911.

The woman's vehicle was found a short time later by deputies on Ashley Crossing Drive. The victim was transported to an area hospital.

With the aid of dogs and a sheriff's helicopter, deputies searched the area but found no sign of the suspect.

He was described as black, about 5-foot-8 and balding, with a medium build and gaps between his teeth. He was last seen wearing a red T-shirt and black pants.