Hugo Osorio Chavez Osorio, 51, is under investigation on suspicion of sex crimes and 13 murders, the country’s attorney general’s office said.
He was initially arrested last Saturday for the murder of a woman and her adult daughter, which he confessed, according to the Associated Press.
The arrest came after neighbors in the western town of Chalchuapa called police and said they heard a woman cry for help.
Officers found the bodies of a 57-year-old woman and her 26-year-old daughter in the house in a pool of blood showing signs of sexual abuse.
The investigations led to searches at various places on the property. Among the bodies found were a 7-year-old girl and two boys aged 2 and 9.
Police did not explain why the number of murders investigated was 13.
Some of the people appear to have been killed around two years ago, the attorney general’s office said on Twitter.
Seven other possible graves on the property are still under investigation. At least 25 people are considered missing in the region.
Chavez Osorio was fired from his police post in 2005 for sexually aggressive behavior and spent five years in prison, investigators said.
Based on information in the case, prosecutors issued arrest warrants for nine other suspects who may have been complicit in 13 murders, including eight women.