(NEXSTAR) — President Joe Biden has ordered flags across the U.S. to fly at half-staff until Monday as a mark of respect for the victims of two shootings in Maine.

In a proclamation Thursday morning, Biden ordered the U.S. flag to fly at half-staff at the White House, public buildings and grounds, military posts and naval stations, and on all naval vessels throughout the country until sunset on Oct. 30.

Authorities were continuing massive search efforts for the man they say shot and killed 18 people and injured 13 others at a bowling alley and restaurant in Lewiston, Maine, late Wednesday.

During a Thursday morning press conference, Maine Gov. Janet Mills said the shooting suspect, Robert Card, is considered armed and dangerous and warns people not to approach him. Card was described as a firearms instructor believed to be in the U.S. Army Reserve and assigned to a training facility in Saco, Maine.

After the shooting, police, many armed with rifles, took up positions while the city descended into eerie quiet — punctuated by occasional sirens — as people hunkered down at home. Schools as far away as Kennebunk, more than 50 miles from Lewiston, closed out of caution on Thursday. And schools and public buildings were closed in Portland, the state’s largest city.

A shelter-in-place advisory was issued Wednesday night for Androscoggin County. That included the community of Lisbon, about 8 miles away, after a “vehicle of interest” was found there, authorities said. It was extended Thursday to Bowdoin, which is in Sagadahoc County.

Mills earlier released a statement echoing instructions for people to shelter. Biden spoke by phone to Mills and the state’s Senate and House members, offering “full federal support in the wake of this horrific attack,” a White House statement said.

This is the 36th mass killing in the United States this year, according to a database maintained by The Associated Press and USA Today in partnership with Northeastern University.

The Associated Press contributed to this report.