OAKLAND -- The estranged husband of a missing mother was arrested Tuesday on suspicion of murder in connection with her disappearance more than a month ago, police said.

Hans Reiser, a man who has recently refused to cooperate with investigators, was arrested Tuesday around 11 a.m. at an acquaintance's home in the 6900 block of Simson Street, according to police.

Nina Reiser, 31, was last seen Sept. 3 while dropping off her 7-year-old son and 5-year-old daughter at her husband's home in the Oakland hills. She never showed up for a meeting with her best friend that evening. Her Honda minivan was found Sept. 9 with her purse and groceries still inside.

Deputy Chief Howard Jordan said that even though police are still looking for Nina Reiser's body they decided to charge Hans Reiser based on biological evidence and statements from friends and relatives of the missing woman.

"Our investigation has gone from a search and rescue to a search and recover," Jordan said at a news conference on Tuesday.

The Reisers have been embroiled in an acrimonious divorce and child custody fight after separating in May 2004. Nina Reiser filed for divorce three months later, but it has not been finalized.

Jordan said Hans Reiser has remained under surveillance for several weeks.

Hans Reiser, who is expected to be charged this week, has refused to speak to authorities. His lawyer, William DuBois, did not immediately return a call Tuesday.

Nina "Nenasha" Reiser moved to the U.S. in 1999 from St. Petersburg, Russia where she worked as a medical doctor. Her mother, who still lives in St. Petersburg, said she talked to her daughter the day she disappeared. She said Hans Reiser has ignored her attempts to speak with him.

The two children remain in protective custody. Their father has tried to convince authorities they should be reunited with him.

Police expressed confidence that the district attorney's office would arraign Reiser on charges for murder even without the discovery of Nina Reiser's body.

"During this investigation we explored all avenues of Ms. Reiser's life, who she had contact with," homicide Lt. Ersie Joyner said. "We interviewed a host of individuals. All avenues all led back to Mr. Reiser being responsible (for) the death and disappearance of Ms. Nina Reiser."

Joyner went on to say that the investigation into a possible homicide began a couple of weeks after she went missing and it included searches of Hans Reiser's home as well as 24-hour surveillance of his activities throughout the Bay Area.

The Reisers' two children, ages 5 and 7, are currently in protective custody. Nina, who is also affectionately called Nenasha, was last seen on Sunday, Sept. 3 dropping them off at their father's house in the Montclair neighborhood of Oakland.

According to a Web site set up to help locate her, police found her purse and groceries from the Berkeley Bowl market inside her car a couple days later.

Investigators originally handled her disappearance as a missing-persons case, but now they say her body is somewhere in the Bay Area.

"It's very important to understand that we had to go to great lengths to be able to prove that Ms. Reiser was a person who had a lot of contact with her family, was financially solvent in the community and all traces of Ms. Reiser being alive or having any contact ceased the day that she went to Mr. Hans Reiser's residence," Joyner said.

Nina Reiser's friend Ellen Doren said Tuesday she didn't want to comment until she knew for sure what charges Hans Reiser would be facing.

Prosecutors are expected to announce their charges by Thursday.

Police told the Oakland Tribune biological evidence that puts the missing woman in a car her husband had access to is a strong part of the circumstantial case against him. They would not say what the biological evidence is, but the term usually includes blood, hair or other body fluids.

The link to the car is important because Nina Reiser's friends have told authorities she would never had voluntarily ridden in a car with him.

Last month, police obtained a search warrant to secure a DNA sample from Hans Reiser.

A $15,000 reward was offered for information about the missing woman.

Reiser's attorney, William Du Bois, was not immediately available for comment. Investigators said today that Reiser has not been questioned as of yet and he has not asked to speak with his attorney.

Police are still asking for the public's help in locating Reiser's body. Anyone with information is urged to contact Sgt. Bruce Brock at (510) 238-3821.