WASHINGTON — The Defense Department’s response to the coronavirus outbreak has expanded to include not only the expected deployment of tens of thousands of National Guardsmen, but also a growing effort to stamp out conspiracy theories that the United States will adopt martial law.

Senior U.S. officials have addressed the issue in briefings, a Pentagon official rebutted speculative online posts, and a new government website is titled “Coronavirus Rumor Control.”

More than 8,000 National Guardsmen were on duty as of Monday to respond to the spread of the virus, with tasks ranging from delivering needed supplies to disinfecting public areas.

Defense Secretary Mark Esper said that President Trump had activated the Guard in California, New York and Washington state in “Title 32” status, in which governors control the forces but the federal government pays for them. Esper quickly added that “this is not a move toward martial law, as some have erroneously claimed.”

The chief of the National Guard Bureau, Gen. Joseph Lengyel, said in a phone call with reporters on Sunday night that he had “seen things on Facebook and the like” that depicted military equipment moving on trains and suggested the Guard was going to launch “some quarantine operation.”

“There is just no truth to this rumor that people are considering, that governors are planning, that anybody is conspiring to use National Guard … to do some sort of a military action to enforce, you know, shelter in place and quarantine,” Lengyel said. “I don’t know how to say that any more clearly than that.”

The Guard has legal authority to participate in law enforcement operations, such as crowd control but usually does so under the supervision of local police, Lengyel said. He left open the possibility that Guardsmen also could take on other missions at the request of local authorities, including stocking shelves in grocery stores if the food supply chain fails.

In tweets, Lengyel said the use of Title 32 status “is a win for all involved” and “no different than when the National Guard responds to natural disasters.”

“Governors and adjutants general, who know best what is needed on the ground, will continue to command Guardsmen and women and use them where they are needed most,” he wrote. “Just as important, the status change also ensures states will be reimbursed by the federal government.”

The discussion is sensitive in a country where martial law has rarely been declared, and has sometimes prompted regrets later, said Phillip Carter, a retired Army officer and analyst with the Rand Corp. who studies civil-military relations. He cited the creation of internment camps, the forced relocation of Japanese-Americans and the enforcement of curfews on the West Coast during World War II as examples.

The issue also is complicated by the variety of missions Guard units have and the many legal authorities under which they operate, Carter said.

Each state and U.S. territory has its own National Guard, and it is often activated by governors under state orders to handle missions during blizzards, hurricanes and flooding. Unlike active-duty forces, Guardsmen usually also have civilian jobs, joining drills to sharpen military skills monthly and attending annual training that is about two weeks long.

Guardsmen can be “federalized” under Title 10 of federal law to carry out missions ordered by the president, but commanders in chief have historically only done so in times of crisis. Examples include president Dwight D. Eisenhower federalizing the Arkansas National Guard to protect black students who were desegregating a high school in Little Rock in 1957 and President Lyndon B. Johnson ordering the Alabama National Guard to protect a civil rights march in 1965.

In more recent years, National Guard units were activated and sent to the southern border under presidents George W. Bush, Barack Obama and Trump, under Title 32 status. That allowed some governors to order Guardsmen from their states back home last year when they determined there was insufficient cause for them to be there.

Historically, activating Guardsmen under Title 32 status has not been controversial because the governors retain control in similar fashion to if the Guardsmen were on state orders, said Steve Vladeck, a constitutional lawyer and professor at the University of Texas School of Law.

“The critical distinction is who is sending orders to the Guard. Is it the governor of the state, or is the president?” Vladeck said.

A fundamental misunderstanding that many people have about domestic use of the military is that it is always tantamount to martial law, Vladeck said.

“Martial law is the extreme case where there is no functioning civil authority,” he said. “It’s not the regular case, where the military is supplementing civil authority.”

That hasn’t stopped rumors from persisting, especially online.

FEMA’s website now includes a page that calls the national lockdown rumor a myth and urges citizens to verify the source of information they share.

In one case last week, an economist, Jim Bianco, posted a video of armored vehicles traveling by train, saying it was recorded on a line north of Chicago and was heading east.

“Coming soon?” Bianco asked vaguely, prompting a backlash from some veterans and responses from others who worried what it meant.

The Pentagon’s top spokesman, Jonathan Rath Hoffman, tweeted Bianco’s video on Saturday and wrote that it was a shipment of new Joint Light Tactical Vehicles, which are made in Oshkosh, Wisconsin. They were traveling to Fort Bragg, North Carolina, Hoffman wrote.

“These deliveries by train to our bases nationwide are not infrequent and have nothing to do w COVID-19,” Hoffman tweeted.

Bianco, reached by email, said that he was “glad the real reason was cleared up” but that his tweet was referring only to the expected activation of more National Guardsmen, something he called a “necessary evil.”

“No regrets,” he wrote. “I posted an accurate video.”