The baby was having trouble breathing when she was taken to the Riverside County Medical Center on Monday. She is not expected to survive.
Police say medical procedures established that her injuries were suspicious.
Officials are not releasing her name as they continue their investigation.
The Moreno Valley Police Department and the Riverside County Sheriff's Department's Homicide Unit are investigating. If you have any information about this case, you're urged to contact Investigator Dean at (951) 955-2777 or Detective Posson at (951) 486-6700.