House Speaker Nancy Pelosi is telling fellow Democrats that the nation’s top military officer has told her there are steps in place that would prevent President Donald Trump from firing nuclear weapons.

Pelosi is holding a conference call with House Democrats about what they should do about Trump doing his waning days in office, including the possibility of impeachment. Democrats and many Republicans blame Trump for this week’s deadly assault on the Capitol by pro-Trump rioters, and many say they are worried that Trump might try something irrational.

Before Friday’s conference call, Pelosi told her colleagues that she had asked Gen. Mark Milley, chairman of the Joint Chiefs of Staff, about preventing Trump from initiating a nuclear strike. A person familiar with Friday’s call says Pelosi has told them that Milley has told her there are precautions in place that would prevent such an action by Trump.

The president has sole authority in the U.S. government to order the launch of a nuclear weapon. But if a military commander were to determine, on advice of his lawyers, that such an order was illegal, then the order could be refused.

It would be illegal to launch a nuclear attack for no reason or as a disproportionate response to a military provocation.

The person described Friday’s conference call on condition of anonymity because the call was limited to House Democrats.