Powerful Democrats in both the House and Senate called Tuesday for an investigation into Drug Enforcement Administration-led operations in Mexico that played a role in triggering violent drug cartel attacks. These attacks left dozens, possibly hundreds, of people dead or missing, including many who had nothing to do with the drug trade.

The call was issued in a letter signed by ranking members of the committees that oversee America’s foreign law enforcement operations and draws heavily on two stories last year by ProPublica and National Geographic that documented the attacks and the DEA’s role. One story reconstructed a 2011 massacre by the Zetas cartel in the Mexican state of Coahuila. It revealed that the wave of killings was unleashed after sensitive information obtained during a DEA operation wound up in the hands of cartel leaders, who ordered a wave of retaliation against suspected traitors.