In shackles and chains for his arraignment Monday, Scott Peterson entered a not guilty plea Monday in the deaths of his pregnant wife Laci and their unborn son, as District Attorney James Brazelton filed murder charges and said he would probably seek the death penalty.

"I am not guilty," said Peterson, 30 (shorn of a recently grown goatee) during the brief legal proceeding in Northern California's Stanislaus County Superior Court.

In the filing, Brazelton said Peterson, a fertilizer salesman, acted "intentionally, deliberately and with premeditation" in killing Laci Peterson, 27, who was eight-months pregnant with Conner Peterson, the couple's unborn child, the Associated Press reports.

The heavily decomposed bodies of Laci and Conner washed ashore last week about three miles from where Peterson said he was fishing in the San Francisco Bay when his wife vanished just before Christmas.

In a highly emotional news conference Monday, Laci Peterson's mother, Sharon Rocha, made her first public remarks since the bodies were found and erupted in tears as she vowed to seek justice for her daughter and the grandson she never knew.

"I can only hope that the sound of Laci's voice, begging for her life, begging for the life of her unborn child, is heard over and over and over again in the mind of that person every day for the rest of his life," she said.

"They didn't deserve to be dumped in the bay and sent to a watery grave."

The charges against Peterson include the special circumstance that he committed more than one murder, allowing prosecutors to seek the death penalty.

Brazelton told ABC News that his office would probably decide by a May 19 hearing whether to seek the death penalty, but he added that he believed it would be likely.