He studies his victims for days, perhaps weeks, learning their names, their addresses, their habits. He finds out when the women will be vulnerable, then attacks them in their homes.

Investigators said Wednesday they believe at least nine rapes and one attempted rape over the past 15 years in Northern California are linked to one man. They say he struck again last month, raping two women here.

During the assaults, some lasting hours, the rapist covers the women's eyes and mouths with duct tape, binds their hands and ankles and talks to them in an easy, conversational tone.

"The words are the same, the duct tape is the same," Davis Police Sgt. Scott Smith said.

Police agencies around the region are trying to find him before he attacks again.

Frustrated detectives have linked several rapes in the 1990s to the same attacker through DNA but have not been able to match it to anyone in DNA databases. One was in the north-central California college town of Chico in 1997.

"I don't know whether this guy has been lucky or good" at concealing his identity, said police Sgt. Dave Barrow.

Investigators think the rapist recently might have made enough mistakes to allow identification. On Oct. 13, he spent six hours raping two women, ages 24 and 28, in their Sacramento home, said Detective Paul Schindler.

One victim walked past his vehicle while it was parked in her garage after he had already broken into the house and assaulted her roommate. A neighbor's security camera caught the same vehicle as it drove by. Police described it as a 2001 or 2002 white Toyota 4-Runner with silver molding, a moon roof and roof rack.

Moreover, the victims were able to give police their first recent description of the rapist.

He has put on 30 to 40 pounds since his attacks began years ago and developed a noticeable pot belly as he has aged, investigators said. He is described as white, 37 to 40 years old, 200 to 250 pounds and 5-foot-8 to 6-foot tall.

More than 30 tips were reported to police after they publicized a police artist's sketch and vehicle description earlier this week, Schindler said. As a result, they also are linking the same man to a 1996 attempted rape in Woodland.

All his known victims since that year have been Asian women who were attacked across a broad swath of Northern California. His first known victims, in Rohnert Park in 1991 and Vallejo in 1992, were white.

During his attacks, the man has threatened his victims with a knife or a gun and often wears disguises.

After raping two women in Davis in 1997, he was photographed wearing a clear, formfitting hockey mask while using one of the victim's debit cards at a Woodland ATM.

On Halloween 1996, he approached a woman's home in Martinez while wearing a skeleton mask. When she opened the door, he forced his way in, raped her and stole her ATM card. About three weeks later, he called her at work to apologize.

"He's a pretty demented person that he would somehow think that the victim would enjoy that," Detective Sgt. Gary Peterson said.

Police are puzzled by the apparent six-year gap between a rape in Davis in 2000 and last month's double rape in Sacramento.

"You can speculate forever on that," said Smith, the Davis police sergeant. "Was he in custody? That doesn't seem likely because we'd have his DNA. Was he out of state? Was he inactive? Or were there more attacks ... and we just don't know about them?"

Other states are checking their own DNA databases and looking for similar rapes, he said.

"The consensus is he didn't just quit for six years," he said.