At least 42 bodies were uncovered over a period of three days from a mass grave near the resort town of Puerto Penasco, officials said on Saturday.

A group of Mexican mothers, known as Madres Buscadoras de Sonora, or the Searching Mothers of Sonora, discovered the mass grave on Thursday, south of the Arizona border at the resort town of Puerto Penasco, also known as Rocky Point.

On Thursday, the group found 13 bodies in the desert near the Gulf of California, also known as the Sea of Cortez. 12 of these bodies were complete skeletons with clothing, while one was a decomposing body, Fox News reported.

On Friday, additional 14 bodies were found and by Saturday, 42 bodies were unearthed from the mass grave. The group of mothers were helped in their search by forensic workers and staff members. The bodies were transported to a lab for further investigation.

The dead have not been identified yet, however, based on the clothing it may seem that two of them are women. It is also unclear for how long they have been dead. Two of the bodies found are still decomposing, suggesting that they may have been recently dumped there.

The cause of death is being determined by the investigators.

"We're just now trying to identify the cause of death," Lupita Orduño, spokeswoman for the state's attorney general's office, said. "We don't know how they were killed. And that's what the forensic lab will do."

The Sinaloa drug cartel is active in the Puerto Penasco area. In 2013, a large shootout had taken place between members of the drug cartel and the military forces in this area.

The group of mothers search for missing persons by investigating reports of mass burial sites that are used by drug cartels to dispose of the people they have killed. According to them, they are only trying to reunite the people with the ones they have lost and looking to catch perpetrators.

“I take comfort in knowing that families are going to be united with their loved one...and that they will have a proper burial, like they deserve,” Fox News quoted Ceci Patricia Flores, the group’s founder and mother of two missing boys as saying.

Such groups, which usually consist of relatives of the missing people, are formed due to poor police investigations.