Experts in artificial intelligence and quantum computing. Specialists on Iran’s nuclear program and Syria’s chemical weapons. Workers helping to relocate Afghans who fled the Taliban.

Democrats on the Senate Foreign Relations Committee grilled a top State Department official on Wednesday at a hearing about the firing last week of more than 1,300 department employees, including longtime policy experts in key national security areas.

Among those fired, said Senator Jeanne Shaheen of New Hampshire, the committee’s top Democrat, were the entire staff of the Office of Casualty Assistance, which supports the families of State Department employees who die while serving abroad.

“The entire team was fired on Friday in the midst of trying to bring back an American citizen who had died overseas,” Ms. Shaheen said. An aide to Ms. Shaheen said the senator was referring to a diplomat killed in a car accident in northern Mexico last week.

The mass layoffs are part of a State Department reorganization implemented by Secretary of State Marco Rubio, who says they are needed to streamline a bloated bureaucracy and root out liberal ideologues.

The firings of U.S.-based workers, along with a roughly equivalent number expected to accept voluntary buyout offers, are meant to shrink the department’s domestic work force of roughly 18,000 by about 15 percent.