The ingredient is helium 3, an unusual form of the element that is formed when tritium, an ingredient of hydrogen bombs, decays. But the government mostly stopped making tritium in 1989.

“I have not heard any explanation of why this was not entirely foreseeable,” said Representative Brad Miller, Democrat of North Carolina, who is the chairman of a House subcommittee that is investigating the problem…

Some government agencies, Mr. Miller said, did anticipate a crisis, but the Homeland Security Department appears not to have gotten the message.

The department had planned a worldwide network using the new detectors, which were supposed to detect plutonium or uranium in shipping containers.