U.S. officials are curiously mum about an alleged al Qaeda plot to bomb commercial airliners in a “spectacular” attack around the Christmas holidays.

The alleged plan was first reported in the British publication Express, citing anonymous security sources. These sources supposedly said that airport officials in the United Kingdom were told to prepare for “the big one,” a bombing by al Qaeda against five planes filled with holiday travelers. And the contours of the alleged attack bore alarming similarities to what U.S. intelligence officials described in September as an al Qaeda plan to bomb commercial airliners using explosive materials that can be hidden in electronic devices and cannot be detected by some airport security technology. Fears that the plot was in the final stages helped prompt U.S. airstrikes against al Qaeda positions in Syria.

But on Monday, American officials weren’t saying whether the latest plot is real or just based on recycled reports from earlier this year. A senior U.S. official told The Daily Beast that the British news report didn’t seem credible but provided no specific information. The publication said the plot was first uncovered by American intelligence. But multiple U.S. officials, including from the Transportation Security Administration, which handles airport security in the United States, referred questions to the Homeland Security Department. Officials there have yet to respond to multiple requests for comment.

The silence was notable. Over the past few months, U.S. officials have been quick to point fingers at an al Qaeda unit in Syria known as the Khorasan Group. The threats from this group are what prompted U.S. airstrikes in September. Those threats also led security officials to enhance certain security measures last summer.

U.S. officials are still on guard against potential bombing attacks, but airport security hasn’t been elevated in the United States. Intelligence officials say that a master bombmaker from al Qaeda’s division in Yemen, Ibrahim Al-Asiri, has trained fellow terrorists in how to make hard-to-detect bombs, which could be hidden inside someone’s body cavity. And a U.S. airstrike in November is believed to have killed David Drugeon, a French jihadi whom intelligence officials say was helping the Khorasan Group to build the sophisticated weapons.

Compounding worries, Khorasan has been attempting to broker an alliance between al Qaeda and ISIS, which to date has shown no indications of plotting to attack inside the United States, counterterrorism officials say. That could change, however, if ISIS obtained the knowledge to build hard-to-detect bombs.

According to the British report, the threat to bomb airliners was deemed so significant that it nearly led to an “outright ban on all hand luggage,” and that mobile electronic devices might still be banned from airline cabins. Scotland Yard has said that it foiled between four and five terrorist plots already this year, and security officials fear an attack by British ISIS members who have trained in Syria to launch attacks inside the U.K.

The threat to British security is arguably greater than in the United States, where only about a dozen individuals are known to have traveled to Syria to fight with ISIS. And some officials have sought to downplay the threat that al Qaeda’s Syrian bomb-making unit actually poses to the United States.

“Khorasan has the desire to attack, though we’re not sure their capabilities match their desire,” a senior U.S. counterterrorism official said in September, shortly after American-led airstrikes in Syria began.