• At least 18 people are dead and 13 injured after a gunman opened fire at a bowling alley and a bar in Lewiston, Maine.
• The suspect in the shooting, Robert Card, remains at large and is considered armed and dangerous, police said.
• Card is a firearms instructor and Army reservist who was committed to a mental health facility over the summer but later released.

Law enforcement agencies are searching for the suspect in a mass shooting that left at least 18 people dead and 13 injured Wednesday at a bowling alley and a bar in the town of Lewiston, Maine.

A warrant has been issued for the arrest of Robert Card, 40, of Bowdoin, Maine on multiple counts of murder. Card remains at large and is considered armed and dangerous, Gov. Janet Mills told reporters at a press conference Thursday. Card should not be approached under any circumstances, Mills said.

The governor said the full weight of her administration is behind the search for Card and that the person responsible for the shootings will be held "accountable under the full force of state and federal law."

Card, 40, is a sergeant first class in the Army Reserve who enlisted in 2002 and serves as a petroleum supply specialist, according to the military. He had no combat deployments.

Card is also a trained firearms instructor, according to a bulletin from the Maine Information and Analysis Center. The rifle used by the suspect was purchased legally this year, two senior law enforcement officials told NBC News.

Over the summer, Card was sent to psychiatric treatment by his military commanders while he was training at the U.S. Military Academy West Point, according to the law enforcement officials and a Defense Department official.

Card's battalion leaders informed staff that he was acting erratically after he claimed to be hearing voices and made threats to the base, the officials told NBC News.

Law enforcement was contacted out of concern for Card's safety, and he was transported by New York State Police to Keller Army Community Hospital for medical evaluation, the Defense Department official said.

Card underwent inpatient treatment for two weeks and was then released, the law enforcement officials said. They did not specify that this treatment took place at Keller Hospital, however.

"While his unit supported West Point summer training, our records indicate he did not instruct nor have any interactions with cadets in training," a West Point official said.

The search is a coordinated effort between federal, state, county and municipal law enforcement. Anyone with information about Card or the shootings should call (207) 213-9526 or (207) 509-9002, police said.

State police have issued a shelter-in-place order for Lewiston and the neighboring towns of Lisbon and Bowdoin as law enforcement personnel search for Card. People should stay in their homes with the doors locked, police said.

The gunman opened fire at Just In Time Recreation bowling alley and Schemengees Bar & Grille, both in Lewiston, on Wednesday evening, leaving seven people dead at the bowling alley and eight people dead at the bar. Three more people were pronounced dead at the hospital. The two businesses are located about 12 minutes apart by car.

Eight victims remained hospitalized on Thursday evening, five of whom were in stable condition and three who were in critical condition, said Steve Littleson, CEO of Central Maine Medical Center.

President Joe Biden has spoken individually with Mills, Sen. Angus King, I-Maine, Sen. Susan Collins, R-Maine, and Rep. Jared Golden, D-Maine, according to the White House. Biden has also offered federal support to state and local law enforcement.

"For countless Americans who have survived gun violence and been traumatized by it, a shooting such as this reopens deep and painful wounds," Biden said in a statement Thursday.

"Far too many Americans have now had a family member killed or injured as a result of gun violence," he added. "That is not normal, and we cannot accept it."

Biden called on Republican lawmakers to work with Democrats to pass a bill banning assault weapons and high-capacity magazines, enact universal background checks, require the safe storage of guns, and end immunity for gunmakers.

At a press conference on Thursday evening, Golden apologized for his previous opposition to an assault weapons ban, and vowed that he would now work to enact such a ban.

"I ask for forgiveness and support as I seek to put an end to these terrible shootings," said Golden, who represents the congressional district in which Lewiston is located.

Speaking at the same event, Collins was asked whether she would also support a ban on assault weapons. The Republican senator said she believed that outlawing high-capacity magazines would be more effective than a ban.

Several states have already attempted to prohibit high-capacity magazines, with varying degrees of success.

Any federal legislation on either assault-style weapons or high-capacity magazines has virtually no chance of becoming law, given the staunch opposition it would face in the Republican-controlled House of Representatives.