An unusually long and large flight restriction has been placed on the Chicago area for several days, a move officials say was sparked by a “credible threat” of attacks on law enforcement.

The Federal Aviation Administration confirmed a temporary flight restriction was put in place “at the request of the Department of Homeland Security.”

The restrictions, which largely target drones, include aircraft from the surface to 400 feet, citing “security reasons.” The restricted zone extends in a 15-nautical-mile radius stretching from downtown Chicago to north of Evanston, west of Elmhurst and south of Dolton.

“Pilots who do not adhere to the following [procedures] may be intercepted, detained and interviewed by law enforcement.

Customs and Border Protection confirmed the restrictions were requested “due to a credible threat of small, unmanned aircraft systems being used against law enforcement during Midway Blitz.”

Midway Blitz is an immigration enforcement operation being conducted in the Chicago area that has led to more than 900 arrests so far, according to DHS.

“The Trump administration will utilize every tool to keep our law enforcement safe,” CBP said in a statement.

The restrictions, which are unusual in both duration and scope, are set to remain in place through Oct. 12.

Similar flight restrictions were put in place in both Los Angeles and Portland, where troops have been deployed, but the coverage area was not as large. Earlier reports indicated another, much smaller, restriction was also in place over Broadview, Illinois, where an ICE processing facility has become the center of several large protests.

The American Civil Liberties Union said the restrictions could “effectively block private citizens and the media from using drones to observe and track ICE raids and arrests.”

“These restrictions appear to follow a disturbing pattern with the Trump Administration — attempting to limit the ability of the public and the media to monitor and demand accountability for their actions,” Ed Yohnka, director of communications and public policy at the ACLU of Illinois, said in a statement. “When redressing claims of the use of excessive force and other abuses by ICE and other Administration agents, video footage gathered by individuals and the media has been crucial. This broad limitation over such a giant swath of the Chicagoland area suggests that ICE intends (as we recently saw) to use their own drones and helicopters for immigration enforcement action. Using these military tactics on civilians is inappropriate, given the attempt to eliminate oversight and accountability, is dangerous to the people targeted in these actions and the entire community.”

While no specifics on the possible threats were given, the Chicago-area order comes as Illinois Gov. JB Pritzker said DHS requested the Department of War send 100 military personnel to Illinois “claiming a need for protection of ICE personnel and facilities.”

U.S. Attorney General Pam Bondi warned last weekend she was sending more federal troops to ICE facilities around the country.

“If you so much as touch one of our federal officers, you will go to prison,” she said.

President Donald Trump on Tuesday proposed using American cities, including Chicago, as training grounds for the armed forces and spoke of needing the U.S. military to combat what he called the “invasion from within.”