Federal officials were forced to cancel naturalization ceremonies this week after discovering a flaw in the immigrant background check system, according to an email obtained by House lawmakers.

“Effective immediately offices are instructed not to approve or oath any naturalization cases in [the Electronic Immigration System],” a U.S. Customs and Immigration Services Official wrote in an email to colleagues. “We have identified an issue with FBI Name Checks initiated through ELIS. At this point we are not confident that proper FBI Name Checks have been run on certain ELIS cases.”