Peace-loving cooperation aside, most cyclists in Portland know exactly who their real enemies are: careless motorists. But a new menace has emerged on the streets with an M.O. that might surprise bikers: This harasser of pedalists rides a bike.

Here's the profile: Usually aboard an older bike with a flat rack, our two-wheeled villain rides up alongside female cyclists and tries to strike up a conversation. The chit-chat quickly moves from strangely friendly—trying to shake hands and exchanging names while riding—to downright confrontational, demanding dates and flipping the women off. The suspect, a white man with black hair, sometimes strikes his victims but never brandishes a weapon.

The trail of clues began in May, when Portland Police Officer Robert Pickett, who works a bike patrol, was trolling the website, where he discovered a posting by a woman who wrote she'd been waylaid by a man on a bike.

"There's no forgetting that guy's face," says the woman, Erica Potechin. She says her assailant rode up next to her on the west side of the Steel Bridge, introduced himself and began asking personal questions as he slowly steered her up against the curb. When she got off her bike, he began yelling, and asked her why girls never wanted to go out with him. He eventually grabbed Potechin's arm and threatened to let the air out of her tires before pedaling away.

The man sounded a lot like someone Pickett had arrested for harassment April 1. Pickett and his partner, Gabe Hertzler, had been flagged down on the Eastbank Esplanade by a runner who said a male cyclist had ridden past her, slapped her on the back and then flipped her off.

Presented with a photo lineup, Potechin identified the same man from the April 1 incident, James Martin Heidtbrink, 39, of Portland. Two other women posting on the site also identified Heidtbrink, who has been jailed for several short stints on charges ranging from domestic violence to disorderly conduct.

After a few weeks of quiet, police say he struck again last month. On July 24, Pickett and Hertzler responded to a 911 call at Southeast 60th Avenue and Salmon Street, where witnesses reported a man on a bike interfering with traffic and harassing female cyclists coming down from Mount Tabor. Once again, Pickett and his partner arrested Heidtbrink.

Pickett believes their suspect, or someone who fits his description, may be responsible for other unreported velo attacks. He hopes to bring enough charges against Heidtbrink to jail him for more than one night, or perhaps get him some mental-health treatment.

The officer asked WW not to publish a mugshot of the suspect in hopes that other witnesses and victims will step forward and pick him out of a lineup.