As many as 4,000 National Guard personnel will be headed to the U.S.-Mexico border, paid for by the federal government, the Associated Press reported Friday night.

Defense Secretary Jim Mattis approved the use of Defense Department funds to pay for the troops to perform border security operations.

“Tonight, National Guard troops are deploying to support border security missions along the U.S. southwest border,” Mattis and Homeland Security Secretary Kirstjen Nielsen said in a joint statement released Friday evening. “Working closely with the border governors, the Department of Homeland Security identified security vulnerabilities that could be addressed by the National Guard.”

Although the federal government will be paying for the operations through September 30, 2018” ― the end of the current fiscal year ― the troops will be under the “command and control of their respective governors,” according to Mattis’ order,

While the National Guard personnel will be performing border security operations, they will not “perform law enforcement activities or interact with migrants or other persons detained by [Department of Homeland Security] personnel” without Mattis’ approval. Arming of the troops “will be limited to circumstances that might require self-defense.”