This week, Sen. Chris Murphy (D-Conn.) staged a 15-hour filibuster on the Senate floor to secure a vote on legislation that would make it harder for people on the terrorist watch list to legally purchase guns in the United States. The effort underscores a strange fact about U.S. firearm laws: Being placed on a federal terrorist watch list is no barrier to passing a gun background check.

“Membership in a terrorist organization does not prohibit a person from possessing firearms or explosives under current federal law,” as the Government Accountability Office concluded in 2010. And indeed, plenty of people on these watch lists do purchase firearms: Between 2004 and 2015, people on the terrorist watch list passed federal gun background checks no fewer than 2,265 times. At least three of those background checks involved the purchase of explosives. Only 212 attempted purchases were blocked, a successful purchase rate of over 90 percent.

Last year alone, people on the terrorist watch list attempted to purchase guns 244 times. Of those, 223 attempts were successful.