Moncada, who did not resist arrest, was taken into custody on administrative immigration violations and was returned under heavy security Friday to Mexico where he faces first degree murder, attempted murder and kidnapping charges.
He was being held at a Tecate, Baja California prison pending court proceedings, according to the Mexican Attorney General's Office.
Moncada told officers he fled to the United States in 1998, illegally crossing the border near San Ysidro, according to the U.S. Immigration and Customs Enforcement.
"This man is suspected of being involved in one of the most heinous mass killings in recent times," Brian DeMore, a field office director for ICE, said in a statement.
Authorities believe Moncada was among several gunmen who raided the compound of an alleged drug trafficker near the Baja California beach resort of Ensenada in 1998. They lined the alleged trafficker and 18 members of his family up against a wall and shot them. Eight children were among those killed.
Prosecutors contend Moncada was a high-ranking member of a Felix Arellano gang, which carried out the killings to prevent the rival gang's marijuana-smuggling operation from becoming too competitive with theirs.
Moncada had been using his real name while living in the U.S., DeMore said.