The hackers who stole data on tens of millions of U.S. insurance holders and government employees in recent months breached another big target at around the same time — United Airlines.

United, the world’s second-largest airline, detected an incursion into its computer systems in May or early June, said several people familiar with the probe. According to three of these people, investigators working with the carrier have linked the attack to a group of China-backed hackers they say are behind several other large heists — including the theft of security-clearance records from the U.S. Office of Personnel Management and medical data from health insurer Anthem Inc.

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 — according to one person familiar with the carrier’s investigation.