MEXICO CITY (AP) — Mexican authorities on Tuesday announced the arrest of six suspected gang members tied to crimes including extortion, an increasingly organized and lucrative side-business for drug cartels.

The governmental National Security Commission said federal police in the southern state of Guerrero seized firearms, ammunition and substances believed to be heroin and marijuana, along with 22 promissory notes from extortion victims.

Gang members have taken to forcing people to sign IOUs for protection payments, going so far as using courts and notary publics to formalize the debts.

In response, some communities formed self-defense militias to resist the gangs' demands.

The Commission said in a statement that the suspects were detained on a highway between the town of Coyuca de Catalan and the coastal resort of Zihuatanejo. It added that one has an outstanding warrant for his arrest in the United States.