Nicole Darrington-Clark
43-year-old Nicole Darrington-Clark sought in connection with stabbing attack. Colton Police Department handout

A 1-year-old girl was slain and her 5-year-old sister and mother critically wounded Monday in a stabbing that Southern California authorities suspect was carried out by the children's grandmother.

Detectives were aggressively searching for 43-year-old Nicole Darrington-Clark in connection with the morning attack, Colton police Cpl. Ray Mendez said.

Investigators do not know the motive for the attack, which took place in an apartment where Clark, her daughter and granddaughters lived together, Mendez said.

Police first said the slain girl was 6 months old, but later said she 18 months old.

Neighbor Patty Williams told the Riverside Press-Enterprise the wounded woman had been "stabbed everywhere."

Another neighbor, Tim Hill, said she ran into his apartment seeking help after the attack. He said he ran upstairs to her apartment and saw the stabbed baby and found her sister in the closet, shaking.

Police decided they couldn't wait for paramedics and took the girl to the hospital, Hill said.

Darrington-Clark should be considered armed and dangerous and may be driving a black Hyundai Sonata, police said.

"I'm sad," Williams said. "I feel like my soul left my body because this is disgusting."

Colton is about 60 miles (97 kilometers) east of Los Angeles.

Darrington-Clark had previously pleaded guilty to stabbing her son and throwing her daughter out of a van while driving on the freeway in 2005. Neither child was seriously injured.

But after the plea, a judge ruled that she was not guilty of the two attempted murders by reason of insanity, the Pasadena Star-News reported in 2007. He sent her to a state mental hospital for 34 years to life, with progress reports required every six months.

It wasn't immediately clear when or why she was released from the mental hospital and whether the daughter in the 2005 attack is the same one critically injured Monday. Police did not immediately reply to a message seeking answers to those questions.

A longtime friend of Darrington-Clark told the Press-Enterprise she was released a few years ago.

"I Facetimed with her a few days ago, and I was worried about her," the friend, Cindy O'Neal, said. "I never thought she would do anything like this. I hope they do find her so she can't hurt anyone else or herself."