Shortly before 11:30 a.m. Thursday, officers responded to a radio call of a child death investigation in the 20000 block of Sherman Way, the Los Angeles Police Department said in a news release.
The boys' mother had "discovered them unresponsive," the news release said.
Firefighter-paramedics responded to the scene and transported both children to a hospital, where one of them was pronounced dead, according to the LAPD. The deceased boy was later identified as Josiah James by the Los Angeles County Department of Medical Examiner.
A preliminary investigation indicated that the children "had ingested, or been exposed to, an unknown substance," the police statement said.
Their mother, Jestice James, was arrested and booked on suspicion of murder. Her bail was set at $2 million. The other residents of the home are not being investigated further, according to the LAPD.
The boys lived at the Palm Vista Apartments, an affordable housing building run by the city of Los Angeles that includes some units for people experiencing homelessness.
Residents of the building say the mother is a known drug user.
Christopher Burko lives at the apartment building and said he attempted CPR on the 3-year-olds.
"They were both laying there lifeless," he said.
The case is being investigated by the Police Department's Juvenile Division/Abused Child Unit.
Anyone with information about the case is urged to contact LAPD Detective Ortiz at (818) 374-5415. During non-business hours or on weekends, calls should be directed to (877) LAPD-24-7 (877-527-3247). Anyone wishing to remain anonymous should call the L.A. Regional Crime Stoppers at (800) 222-TIPS (800-222-8477) or visit lacrimestoppers.org.