There is "no question" that the female body found in the San Francisco Bay this week is Laci Peterson, California Attorney General Bill Lockyer said in a news conference Friday.
Lockyer also announced that DNA testing established that the fetus found is the child of Scott and Laci Peterson.
"We're scientifically convinced the match is one in billions," Lockyer said.
Earlier in the day, Scott Peterson was arrested without incident in San Diego in connection with his wife's disappearance, a San Diego County Sheriff's deputy said. Sources said police had been aware of Peterson's location for the past few days. Peterson was considered a flight risk, and had indicated that he had discovered he was under police surveillance, Lockyer said.
Prosecutors said Peterson was expected to be arraigned early next week. "He'll be charged with capital murder," with the option of seeking the death penalty, said Stanislaus County District Attorney Jim Brazelton. "There are no other suspects in this case," he added.
They said they would seek a special circumstance of double homicide, which could carry the death penalty upon conviction. They hadn't decided, however, whether they would seek the death penalty.
Peterson was being driven to the Stanislaus County jail in Northern California on Friday night for a court appearance next week.
The bodies of a male fetus and a woman were discovered about a mile apart on Sunday and Monday by people strolling along the San Francisco Bay shoreline.
Law enforcement sources have told Fox News that ever since the case first opened they believed Peterson was responsible for the murder. They suspected him of throwing his wife's body into San Francisco Bay and securing it with concrete blocks, hooks and fishing weights.
Scott Peterson has denied any role in his wife's disappearance.
The state's DNA laboratory in Richmond worked "24-seven" to determine if the remains discovered are those of Laci Peterson and the son she was due to bear in February, said Hallye Jordan, a spokeswoman for the state attorney general's office.
Crime lab technicians compared DNA swabbed from the mouths of Laci Peterson's parents and taken from Laci Peterson's hairbrush with samples from the two corpses.
It was not immediately known how Peterson delivered the baby. One theory centers around the unusual medical phenomenon called "coffin birth." It occurs when the gas that naturally builds up in the abdomen and pelvic area of a decomposing body produces enough pressure to push the unborn baby through the birth canal.
Meanwhile, Kirk McAllister, Scott Peterson's attorney, says his client still hopes his missing wife and son will be found alive.
McAllister told the Modesto Bee he had spoken regularly with Peterson this week.
"His thought is that certainly, for somebody, this represents a tragedy," McAllister said of the discovery of the bodies. "He is hoping this doesn't mean that his search for Laci and the baby is over."
Laci Peterson, 27, was eight months pregnant when she vanished from her Modesto home on Christmas Eve.
Her husband told police he last saw her that morning as he left to go fishing in Berkeley, three miles south of where the bodies surfaced.
Modesto police seized his boat, pickup truck and nearly 100 items from the couple's house but had not formally named him as a suspect in his wife's disappearance.
A knock at the door went unanswered Friday evening at the home of Scott Peterson's parents in Solana Beach, north of San Diego.
"Families in their circumstances will always tell you the worst thing is not knowing," said spokeswoman Kim Petersen, executive director of the Carole Sund-Carrington Memorial Reward Foundation. "I don't know if relief is the right word. ... The waiting this week has been horrific for them."
From virtually the moment his wife was reported missing, Scott Peterson's moves and statements have been scrutinized by authorities.
Peterson traded in his wife's Land Rover for a new pickup truck, considered selling their home and eventually admitted an extramarital affair with a Fresno woman while his wife was pregnant with the couple's baby.
Massage therapist Amber Frey said she became Scott Peterson's girlfriend after he told her he wasn't married.
Shortly after, Peterson said he'd told his wife about the affair in the days before she vanished.
"It (the affair) was not a positive, obviously ... but it was not something that we weren't dealing with," he said during a television interview. "It wasn't anything that would break us apart."
The affair turned Laci Peterson's family against the son-in-law they had earlier supported. They begged him to cooperate with Modesto police, who had labeled him "uncooperative."
Scott Peterson launched his own search effort, separate from the one organized by his wife's family and sanctioned by police. At one point, as searchers looked in the San Francisco Bay and around Modesto, Scott Peterson showed up in Los Angeles to distribute fliers to volunteers at a local hotel.
"We simply have to expand the geographical area," he said at the time.
In February, Scott Peterson said in an interview that he missed his wife and the child she was to bear.
"I can't drive. I can't sleep," he said then. "Sometimes I feel I just can't do it. I feel like I'm in a dark corner and I just can't function."
Laci Peterson's biological father, Dennis Rocha, had told a Boston television station that he thinks it's only a matter of time before his son-in-law is charged in the case.
"I just can't see Scott being out there free," Rocha said. "That would just eat me up."
Scott Peterson's father, Lee, had said he didn't want to comment on the suspicion swirling around his son, but acknowledged that the last few days had been difficult for his family.
If the woman's body is not identified as Peterson, the lab will begin comparing the samples with likely matches in a database of 100 DNA samples of other missing people or their relatives. The state has more than 25,000 active missing person cases.
Fox News' Rita Cosby and The Associated Press contributed to this report.