A Greyhound bus station in Phoenix has been turned into a refuge for undocumented immigrants dropped off by the Department of Homeland Security.

Local Fox affiliate KSAZ-TV was told by federal officials that the immigrant drop-offs are part of a larger operation. The operation involves moving hundreds of undocumented workers who were detained in Texas to Arizona, because officials in Texas cannot process them all. Immigrants dropped off in Phoenix are supposed to report to a local Immigration and Customs Enforcement office within 15 days of reaching their destination.

A Greyhound spokesperson told KSAZ that this is the first time immigration officials have dropped off loads of undocumented immigrants, and that they did not receive much notice before the large groups were dropped off. Volunteers, immigration officials, and Greyhound have been providing some food and water to those delivered to the station. Greyhound is also allowing the immigrants to use phones to call family or friends to get tickets for traveling.

Arizona Gov. Jan Brewer, a Republican, sent a strongly worded letter to President Obama on Monday in response to the offloading of immigrants in her state, demanding an explanation and help securing the U.S.-Mexico border.

In it, she writes she is "deeply concerned about this troubling policy" and that "it is especially disconcerting that DHS instituted this operation without any notification" to the governor, Arizona law enforcement or the state's Department of Homeland Security.

Brewer continued by calling the operation "another disturbing example of a deliberate failure to enforce border security policies and repair a broken immigration system," and urged Obama to "end this dangerous and unconscionable policy immediately, and instead take actions to fulfill the federal government's fundamental responsibility of protecting our homeland by securing our nation's borders."