An Oakland man who left DNA behind after what police said were three unwanted sexual encounters on public transit buses two and three years ago is only in custody now because of a DNA sample he was required to provide the state Department of Justice in a more recent, unrelated arrest, authorities said.

Police in Rohnert Park, Novato and San Francisco already knew all three had submitted semen samples with the same DNA in the bus cases. But they couldn't put a name to it until they were notified in January of a "hit" in the state DNA database, Rohnert Park Public Safety Sgt. Jeff Nicks said.

The DNA was linked to an Oakland man named Perry George Roberson, 58, who was arrested in September for suspected resisting arrest, authorities said.

No charges were filed in the case, but Roberson was required to submit DNA that subsequently led to his arrest in the July 1, 2009, Rohnert Park case involving a 15-year-old Santa Rosa girl, Nicks said.

Echoing the stories of another victim in Novato on the same day and a woman in San Francisco who experienced something similar the following year, the girl said she was riding a Sonoma County Transit Bus from Santa Rosa Junior College to Petaluma when a man got on, sat next to her and pushed himself against her until she suddenly realized he had been masturbating, Nicks said.

She was too upset and afraid to make any commotion, and apparently no one else on the bus realized what had happened, Nicks said. But the girl and her mother reported the incident afterward, and Rohnert Park police took up the case because it was believed to have happened in their jurisdiction, Nicks said.

Like police in Novato and San Francisco, Rohnert Park investigators submitted semen samples to be run in the state database, but at that time there was no way to identify where it came from.

But DNA samples get run through the database periodically in hopes of an eventual hit like the one that landed Roberson in jail, Nicks said.

He said the Santa Rosa victim, now an adult, identified Roberson from a photo lineup before investigators obtained a warrant for his arrest.

Roberson was taken into custody at his home on Wednesday and booked into the Sonoma County Jail for suspicion of sexual battery, lewd acts in public and lewd acts with a child. His bail is $50,000.

(You can reach Staff Writer Mary Callahan at 521-5249 or