The 17-year-old from Moreno Valley went to summer school Thursday morning at Valley View High School. Lopez was supposed to meet her sister and a friend at home but did not return.
Some of her belongings were found in a field Lopez often used as a shortcut to her home, authorities said.
Police said there were signs of a possible struggle.
Authorities said Lopez had no apparent problems at home and didn't think she had just run away.
"We do not believe she's a runaway. If we did believe that, we wouldn't be here today," Riverside County sheriff's Sgt. Joe Borja told reporters at a news conference Friday. "If Norma's out there, we're looking for you. If you haven't been taken, please call. We're worried about you."
They're also looking for a green, newer model sport utility vehicle that was seen speeding from the area.
Anyone with information is urged to call Moreno Valley Police at (877) 242-4345.
The Associated Press contributed to this report.