WASHINGTON — Aaron Hull, head of the Border Patrol's El Paso Sector, will be reassigned to Detroit on Monday, according to an internal document obtained by NBC News, amid controversy over the treatment of immigrants in detention in and around El Paso, Texas.

The reason for Hull's reassignment was not immediately clear, but it follows a series of reports about unsafe and unsanitary conditions in border stations in the El Paso sector. Most notably, lawyers serving as monitors for a federal court order reported that children being held in the border station in Clint, Texas, which is part of the El Paso sector, were going weeks without showers, living in soiled clothing and having to take care of toddlers who were being neglected by agents.

Asked by NBC News in June about the reports alleging mistreatment of children in his facilities, Hull said, "It’s hurtful. We have agents risking their lives to enforce the rule of law."

Hull said El Paso Sector agents had performed well, given that El Paso had gone from being the seventh busiest Border Patrol sector to the second busiest in ninth months. Said Hull, “I’m very impressed with these El Paso Sector agents to go basically from a walking pace to a sprinting pace.”

Under Hull's leadership, Customs and Border Protection moved children out of the Clint station into a new soft-sided tent facility, then began moving some children back the next day.

One Department of Homeland Security official said Hull is known as a "law and order" Border Patrol chief and often acts autonomously without the direction of his superiors.

Agents in Hull's sector also kept immigrants in a "holding pen" under a bridge in El Paso in March, according to several media reports.

Chief Patrol Agent Gloria Chavez, currently in charge of the El Centro Sector in California, will replace Hull as chief of the El Paso sector until further notice, the document said.

Customs and Border Protection did not immediately respond to a request for comment.