Republican presidential nominee Donald Trump speaks at a campaign rally in West Palm Beach, Fla., on Oct. 13. (Mike Segar/Reuters)

Ten former nuclear launch control officers who once held the keys needed to fire on the president’s order have signed an open letter saying they think Donald Trump should not be entrusted with the nation’s nuclear codes.

The letter, issued Thursday, says the decision to use nuclear weapons requires “composure, judgment, restraint and diplomatic skill” — all qualities that the former Air Force officers who signed it said Trump lacks.

“On the contrary, he has shown himself time and again to be easily baited and quick to lash out, dismissive of expert consultation and ill-informed of even basic military and international affairs — including, most especially, nuclear weapons,” the letter says. “Donald Trump should not be the nation’s commander-in-chief. He should not be entrusted with the nuclear launch codes. He should not have his finger on the button.”

The letter is the latest in an extraordinary series of missives signed this year by diplomats and national security experts warning of the dangers they think a Trump presidency would pose. Last month, in a break from the trend, 88 retired military leaders endorsed the Republican presidential nominee. But most of the letters have reflected the views of those who consider Trump unfit to be commander in chief.

The former missileers who signed Thursday’s letter served at the nation’s four underground launch centers in the Great Plains from as long ago as the 1960s to, most recently, 2013. They do not endorse Democratic nominee Hillary Clinton, or even mention her name, in the letter.

“No one who signed the letter wanted to disclose a position who they would vote for,” said Bruce Blair, who organized the letter and said he is a Democrat who will vote for Clinton but has no idea which candidate the others support. “The letter is focused entirely on Trump.”

Blair said the signatories are part of a circle of friends and acquaintances who were “flabbergasted and flummoxed” by the prospect of Trump winning the election. He said they decided several weeks ago to release an open letter, when polls showed Trump and Clinton much closer than more recent numbers do.

“It sort of snuck up on us,” he said. “They were at a virtual tie in the polls. You kind of woke up one day and realized it’s very possible that this person, this pathological liar, who lacks self-control, who knows very little about anything and is angry and aggressive, who might lash out with nuclear weapons if he’s elected, could win. That sort of dawned on everyone. We felt we needed to weigh in.”

The nation’s nuclear-armed missiles are kept at Air Force bases in Cheyenne, Wyo., Minot, N.D., and Great Falls, Mont. A fourth base’s nuclear facility, in Grand Forks, N.D., has been decommissioned.

If a president orders a missile launch, Blair said, five crews are equipped with keys used to fire a total of 50 missiles. Although the keys are interconnected, the missiles can be launched if just two crews carry out the order.

“Only the president can order a nuclear launch,” the letter states. “That order cannot be vetoed and once the missiles have been launched, they cannot be called back. The consequences of miscalculation, impulsive decision-making or poor judgment on the part of the president could be catastrophic.”

Blair said the officers are trained to put aside any personal doubts and trust in the system and the leadership.

“The presumption is, the commander in chief is acting in the national interest, and his decision should be grounded in knowledge and good advice,” he said. “Everyone would have to assume that, even though they had doubts — very strong doubts, if Trump were president.”