Two more bodies have been discovered at a Tijuana, Mexico, property where investigators earlier found the remains of a missing California couple buried under the dirt floor of a house on Friday.

Jesús Rubén López Guillén, 70, a U.S. resident, and his wife Maria Teresa Guillén, 65, a naturalized U.S. citizen, were reported missing by their daughter Norma López after they traveled from Garden Grove to Tijuana on Jan. 10 to collect more than $6,400 in overdue rent from their 37-year-old son-in-law.

Police in Garden Grove launched a missing persons investigation after López said she could no longer track her parents’ movements through the Find My Phone app. She said the last signal she received before their phone went dead was at the property they owned where her husband was living in southern Tijuana, about 4 miles from the U.S.-Mexico border. Their bodies were found buried under the dirt floor of one of the property’s three homes late Friday.

While conducting an investigation into the circumstances of the Guilléns’ murder, Mexican investigators say they discovered the bodies of another couple buried in the property. It is not known if they were found in the same house as the Guilléns’ remains. The new victims have not yet been identified, but police in Mexico say they also may have been involved in a monetary dispute with the son-in-law.

The son-in-law, a Mexican national who was deported from the U.S. in 2012 and identified only as “Santiago” in court documents, was first charged with the California couple’s disappearance and taken into custody while the property was searched. Baja California state prosecutor Hirán Sánchez confirmed that when his in-law’s bodies were found, he was charged with their murder.

Sanchez told reporters that when the son-in-law was first questioned about what happened to his in-laws, he offered up a “series of contradictions” including a tale that they had walked across the border and that he had picked them up. López says her parents had instead driven their own pickup truck to retrieve the money.

The son-in-law also told police that he first took them to their property and then they went together to a bank to exchange currency he paid them, after which he said he drove them back to the border. Instead investigators say that the son-in-law tried to extract money with the couple’s bank cards.

“The Guilléns drove themselves to their houses, not Santiago,” Sanchez said at a news conference. “They never left.”