U.S. immigration officials are planning a month-long series of raids in May and June to deport hundreds of Central American mothers and children found to have entered the country illegally, according to sources and an internal document seen by Reuters.

The operation would likely be the largest deportation sweep targeting immigrant families by the administration of President Barack Obama this year after a similar drive over two days in January that focused on Georgia, Texas, and North Carolina.

Those raids, which resulted in the detention of 121 people, mostly women and children, sparked an outcry from immigration advocates and criticism from some Democrats, including the party’s presidential election frontrunner Hillary Clinton.

 Immigration and Customs Enforcement (ICE) has now told field offices nationwide to launch a 30-day “surge” of arrests focused on mothers and children who have already been told to leave the United States, the document seen by Reuters said. The operation would also cover minors who have entered the country without a guardian and since turned 18 years of age, the document said. Two sources confirmed the details of the plan.

The exact dates of the latest series of raids were not known and the details of the operation could change.

The operation in January marked a departure for ICE, part of the Department of Homeland Security, from one-off deportations to high-profile raids meant to deter migrants from coming to the United States.

An ICE spokeswoman said the agency does not “confirm or deny the existence of specific ongoing or future law enforcement actions.” The spokeswoman said immigrants who arrived illegally after Jan. 1, 2014 are priorities for removal.

Federal resources were strained in 2014 under a wave of illegal migrants crossing the U.S.-Mexico border, especially women and children fleeing violence […]