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LOS ANGELES (AP) _ The mother of a man accused of sexually assaulting 31 women apologized Tuesday for her son, a drifter who often stayed with friends and bicycled through the beachside neighborhood where many of the attacks occurred.

``I cannot believe he did it, but the evidence said he did,'' Alice Rathbun said. ``On behalf of my son, I want to apologize to all these women who have had this frightening experience.''

Prosecutor Rich Goul said he expected charges to be filed Wednesday against Mark Wayne Rathbun, 32, who was arrested Sunday.

Police said DNA evidence linked Rathbun to 13 of the sexual assaults that occurred over the last six years, and there were similarities between those cases and 18 others. The first two attacks occurred in Seattle, the rest in the Long Beach area.

The victims ranged in age from their 30s to their 80s. Nearly all of them were home alone at the time of the attacks, suggesting the assailant planned each crime, police said Tuesday.

Alice Rathbun, 75, lives three miles from the Belmont Shore area of Long Beach, where many of the attacks occurred. Sometimes her son lived with her, and sometimes he stayed with friends, she said in a telephone interview.

She said her son _ an only child whose father was killed in Vietnam in 1969 _ spent time in a youth camp after being arrested for trespassing and burglary. After the alleged theft, a priest told Rathbun he should see a psychiatrist, the mother said.

While talking to her son after his arrest, Rathbun said she asked, ``'Why did you do it?' and he didn't answer directly.''

Goul said Tuesday that an attorney had not been assigned to Rathbun.