United Airlines customer flight records have been lost in a data breach, it has been reported.

According to Bloomberg the breach was detected in May or June, and involved flight manifests. Bloomberg reports that the the same attackers who stole 21.5 million Social Security numbers and background check information the US government were behind the attack.

“The previously unreported United breach raises the possibility that the hackers now have data on the movements of millions of Americans, adding airlines to a growing list of strategic U.S. industries and institutions that have been compromised. Among the cache of data stolen from United are manifests -- which include information on flights’ passengers, origins and destinations," said the Bloomberg report.

The attack could mean that the hacking group could be planning a massive cross-referencing effort, according to other reports. Computerworld said that they could be “amassing a vast database of files that could be cross-referenced to identify US citizens working in defence and intelligence with security clearance who might be open to recruitment or vulnerable to blackmail.”

A United spokesman said the report was based on speculation and that United customers' private information is safe, but declined to confirm that a breach occurred.

As We Live Security reported recently, United Airlines paid out two million flight miles to two security researchers who uncovered 14 separate vulnerabilities in the company’s operation just days ago.