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“We can never guarantee 100 percent security in our transportation systems, whether it’s rail or maritime or aviation. They’re always going to be exploitable,” Mr. Nelson said.

He added that it’s common for government agencies to use contractors to conduct background checks, and no amount of vetting can prevent a security breach, as in the case of former National Security Agency contractor Edward Snowden.

“You do everything you’re supposed to do, but you can never eliminate all of the risk,” Mr. Nelson said. “There’s always going to be some margin of error.”