Muslim people and those perceived to be Muslim are increasingly at risk of being racially profiled on airplanes and in some cases forced to leave the aircraft, according to a joint statement released Wednesday by Muslim Advocates and the NAACP Legal Defense Fund. The Department of Transportation should investigate each case in which a passenger was forcibly asked to leave an airplane, the civil rights groups said.

These incidents and countless others not only violate federal law but also send a message to victims that they are second-class passengers,” read an excerpt from the statement, adding, “Muslims and people of color — like all other law-abiding travelers — should be able to travel for any purpose without being discriminated against because of their appearance or personal religious beliefs.”

At least 11 incidents occurred in the past six months, with five incidents occurring in April alone, the statement said. In one highly publicized incident in March, an Arab-American family was asked to leave a plane headed from Chicago to Washington. The flight attendant did not give the Shebley family a reason for the request, saying only that authorities were “investigating.” The family was asked to leave for “no reason [other] than how we look,” wrote Eaman-Amy Saad Shebley in a Facebook post that has now been shared more than 54,000 times. The airline booked the family on another flight and said they were asked to leave because of an issue with the child’s safety seat.

Airplane security was heightened in the U.S. after Islamic terrorists hijacked four planes on Sept. 11, 2001, and crashed them into the World Trade Center towers in New York and the Pentagon in Washington, killing nearly 3,000 people. Following terror attacks in Paris, Brussels, London and Spain in the 15 years since, fears concerning terrorism have mounted. It’s not the first time civil rights advocates have accused air safety officials and airlines of discriminating against Muslims, and the inspector general of the Department of Homeland Security announced earlier this month it would examine claims of racial profiling allegedly carried out by the Transportation Security Administration.