An internal NTSB memo informed employees Friday that they would not be eligible for the program.

NTSB, an independent and apolitical agency that does not report to the executive branch, investigates transportation accidents, provides aid to those affected by them and recommends safety measures.

That message, dated 9.41 pm (local time) Tuesday, went out barely 23 hours before an American Airlines regional jet operated by PSA Airlines and a US Army Blackhawk helicopter collided in a tremendous fireball, plunging all 67 on board the two aircraft into the icy Potomac River below. There were no survivors.

The disaster apparently left top brass at the agency scrambling behind the scenes to find a way to exempt employees from the Trump administration’s offer.

On Friday, the head of the NTSB sent a message to any employee who agreed to administration’s initial message to “rescind their deferred resignation letter immediately” with the Office of Personnel Management.

One source said the initial message came as a shock to staffers, including highly specialised investigators, many of whom are nearing retirement. “It’s not like we have an easy time finding people,” the source said.

Another source told CNN they know investigators who were seriously considering accepting the offer.

NTSB Chair Jennifer Homendy sent an agencywide email on Friday saying the agency was “granted a full exemption” to the deferred resignation program.

The move “means this program is not available to NTSB employees.”

“Given the nature of our safety work, limited budget, and the ongoing investigation of the highest fatality, mass casualty aviation accident since 9/11,” said the email message viewed by CNN, “we need each and every position represented at our agency in order for us to successfully carry out our mission critical work.”