North Carolina Republican Sen. Thom Tillis says he made a mistake by taking off his face mask indoors last month at a White House event. Many attendees — including Tillis and President Donald Trump — later tested positive for COVID-19.

The senator, who has been a strong supporter of wearing masks in public, was wearing one outside at the White House Rose Garden on Sept. 26, when Trump nominated Amy Coney Barrett to the Supreme Court. But photos showed him without one at an indoor reception later.

“I have to admit that I let my guard down because we’d all been tested about two hours before the event,” Tillis told WRAL-TV in an interview Tuesday. “It’s just another experience that tells me, even when you think you’re in a safe setting, you should always wear a mask.”

Tillis, who is running for reelection next month against Democrat Cal Cunningham, announced his positive test last Friday and has been quarantined at his North Carolina home. He had already apologized in late August when he took off his mask while in a crowd of people attending Trump’s GOP nomination speech outside the White House.

Many people who attended Barrett’s nomination in the Rose Garden tested positive. Tillis said the president called him on Monday after Trump was released from the hospital, where he received three days of treatment. He said the two discussed their infections and Barrett’s confirmation.

During virtual phone meetings this week with constituents about the coronavirus, Tillis said that he had experienced mild symptoms that had since disappeared. He said he hoped to return to Washington next week.

Tillis said he doesn’t believe he contracted the virus at the event because his symptoms didn’t surface until seven days later.

“That doesn’t excuse the fact that, in every case, we should wear a mask because scientists tell us to do it,” he told WRAL on Tuesday.

The Centers for Disease Control notes that the incubation period for COVID-19 is thought to extend to 14 days, with a median time of four to five days from exposure to symptoms onset.

All of Tillis’ campaign staffers in close proximity to him last week have tested negative for the virus so far, his campaign said on Tuesday. His Charlotte campaign headquarters is closed through the end of the week as a precaution.

Photo via AP Photo/Gerry Broome.