The second set of bodies — one male and the other female — was discovered in a state of advanced decomposition, the attorney general’s office for the state of Baja California, just south of San Diego, said late Saturday.
All four bodies were covered in lime when found, investigators said.
Jesus Ruben Lopez Guillen, 70, and his wife Maria Teresa Lopez, 65, of Garden Grove, Calif., a couple with dual U.S.-Mexico citizenship, vanished Jan. 10 after they crossed the border to collect more than $6,700 in rent from tenants of two houses they owned in Tijuana.
Their bodies turned up in one of the houses, the San Diego Union-Tribune reported, citing Mexican investigators.
Their daughter, Norma Lopez, who lived with them in California, reported them missing.
The suspect, known as Santiago N., 36, had a criminal record in the U.S., and was deported in 2012.
He had been living at the property in Tijuana and supposedly had collected rent on their behalf.
The motive for the killings appeared to be a dispute over rent payments, The Associated Press reported.
Investigators said Santiago confessed that he buried them on one of their properties.
The Associated Press contributed to this report.