The US government plans to turn an empty 55,000-square-foot warehouse in south Texas into a processing facility for unaccompanied children who have entered the country illegally, according to construction permits.

The permits, obtained by the Associated Press on Monday through a public records request, reveal plans for four fence-enclosed pods inside a corrugated steel warehouse in McAllen that could eventually accommodate about 1,000 children. The Department of Homeland Security did not immediately respond to a request for comment.

More than 52,000 unaccompanied minors have been arrested since October after entering the United States illegally, a 99% over the same period a year earlier. About three-quarters of those children have been arrested in the Rio Grande Valley in south Texas.

US law requires that the children be processed and transferred to custody of the Health and Human Services Department within 72 hours of their arrest. The wave of children has overwhelmed Border Patrol stations in South Texas that are ill-equipped to house children for an extended period of time, so Homeland Security has been flying planeloads of children to Arizona for processing at a facility in Nogales before sending them to shelters around the country.

The new processing facility would be less than a mile from the Rio Grande Valley's busiest Border Patrol station. Floor plans show rows of cells with unsecured doors on either side of open "interaction/play" areas. Boys and girls would be separated and portable toilets would be installed.

"Minors will be staged here until processing is completed and then they (will be) moved to a different location," according to a fire protection engineering analysis submitted by the US General Services Administration.

That agency, which provides buildings for government operations, signed a one-year lease for the property.

The documents do not indicate when renovations would be complete. City attorney Kevin Pagan said McAllen had expedited the permitting process.

Most of the children are from Honduras, Guatemala and El Salvador. Many are fleeing widespread gang violence, and some are looking to reunite with parents already in the United States.

Homeland Security Secretary Jeh Johnson wrote an open letter to Central American parents warning them of the dangers of such a journey and emphasizing that the children could be deported. "The criminal smuggling networks that you pay to deliver your child to the United States have no regard for his or her safety and well-being - to them, your child is a commodity to be exchanged for a payment," the letter said.