Dozens more people were reported wounded in the rampage in the small town of Lewiston and, in the absence of a formal death toll, local officials estimated the number of fatalities at between 16 and 22.
Lewiston was in lockdown Thursday morning, with schools closed and residents ordered to stay indoors.
Eighteen people were killed and another 13 wounded in the mass shooting in Maine, Governor Janet Mills said Thursday.
"I am profoundly saddened to stand before you today to report that 18 people lost their lives and 13 people injured in last night's attacks,” Mills said, as police kept searching for the gunman in the Wednesday night rampage at a bowling alley and a bar.
Police said a man named Robert Card -- seen in surveillance footage pointing a semi-automatic rifle with an extended clip as he walked into the bowling alley in Lewiston -- should be considered armed and dangerous.
News outlets had broadcast footage of people fleeing in terror from the venue after the shooting started on Wednesday evening.
One survivor whose name was not given told CNN he was 15 feet (5 meters) from the gunman when he opened fire. He thought at first it was a balloon popping.
"And as soon as I turned and saw it was not a balloon and he was holding a weapon, I just booked it down the lane and I slid basically into where the pins are and climbed up into the machine and was on top of the machines for about 10 minutes until the cops got there," he told CNN.
Card is a certified firearms instructor and a member of the US Army Reserve, according to multiple law enforcement sources.
Maine public safety official Mike Sauschuck said he was not prepared to give a death toll, calling it "a very fluid situation."
He told reporters that police were flooding the streets in search of the gunman.
"We have literally hundreds of police officers working around the state of Maine to investigate this case, to locate Mr. Card," he said.
President Joe Biden made calls -- stepping away from a state dinner honoring Australia's prime minister -- to Maine's governor, its two senators and a local congressman to offer federal support, the White House said.
- Multiple locations -
Police and rescuers reportedly arrived at the Sparetime Recreation bowling alley at about 7:15 pm (00:15 Thursday GMT) in response to an active shooter, and then received reports of another shooting at the Schemengees Bar & Grille.
Police issued photographs of 40-year-old Card at the bowling alley, where he appears calm and composed as he moves through the doorway with his rifle raised.
Another survivor, Riley Dumont, said she and her family fled to a corner of the bowling venue and hid behind tables and a bench.
"I was laying on top of my daughter. My mother was laying on top of me," Dumont told ABC News.
"It felt like it lasted a lifetime," she added. "I just remember people sobbing and crying."
Sauschuck said officers had located a "vehicle of interest" they had been looking for -- a white sport utility vehicle (SUV) -- in Lisbon, a town around eight miles (12 kilometers) from Lewiston, where residents had also been warned to stay off the streets.
Card was not in the vehicle, reports said.
- 'My hometown' -
The shooting is one of the deadliest since 2017, when a gunman opened fire on a crowded music festival in Las Vegas, killing about 60 people.
Gun violence is alarmingly common in the United States, a country where there are more guns than people and where attempts to clamp down on their spread are always met with stiff resistance.
The United States has recorded at least 565 mass shootings this year, according to the Gun Violence Archive (GVA), a non-governmental organization that defines a mass shooting as four or more people wounded or killed.
Wednesday's attack was the deadliest mass shooting in 2023 so far, according to the GVA's data.
Efforts to tighten gun controls have for years run up against opposition from Republicans, staunch defenders of the constitutional right to bear arms.
The political paralysis endures despite widespread outrage over recurring shootings.
Lewiston is the second most populous city in Maine located some 30 miles north of the largest city, Portland.
Robert Card: US soldier suspected in shooting rampage
Robert Card, a 40-year-old soldier in the US Army Reserve, is suspected of gunning down 18 people in a small town in the northern state of Maine.
The shooting -- which targeted a bowling alley and a bar in Lewiston on Wednesday night -- is one of the deadliest since 2017, when a gunman opened fire on a crowded music festival in Las Vegas, killing about 60 people.
Authorities have issued a warrant to arrest Card for murder, Colonel William Ross of the Maine State Police told a press conference Thursday, adding that he is considered "armed and dangerous" and warning people not to approach him.
The Lewiston police department released images of a bearded man dressed in a brown hoodie and blue cargo pants and armed with an assault rifle.
Card, born in 1983, is a certified firearms instructor and a member of the US Army Reserve, according to multiple law enforcement sources.
The US Army provided AFP with details on Card's military service, saying he is a sergeant first class in the Army Reserve with "no combat deployments" who enlisted in December 2002.
Card is a Petroleum Supply Specialist, which entails supplying the Army "with the fuel it needs to maintain a state of readiness at all times," according to a US military career website.
- 'Mental health issues' -
"You'll supervise and manage the reception, storage, and shipping of bulk or packaged petroleum-based products. You'll oversee its use, dispense it to various vehicles and aircraft, and make sure it is being transported and handled safely," it says.
The information on Card's service history did not mention him being a firearms instructor, though it is possible he was in a civilian rather than military capacity.
According to ABC News, Card spent two weeks at a mental health facility earlier this year after allegedly threatening to carry out a shooting at a National Guard facility.
NBC News also referred to a threat to a National Guard installation, citing a law enforcement bulletin as saying Card "recently reported mental health issues to include hearing voices and threats to shoot up the National Guard Base" in Saco, Maine.
Officials at Thursday's press conference would not elaborate on the reports, however, saying they did not yet know the motive behind the killings.
Maine resident Liam Kent told NBC that he grew up near Card and his family, who he described as "gun fanatics."
"They for all intents and purposes are very much associated with right-wing militias. It's known in the town to stay away from them and to not approach them," Kent said.
Biden condemns 'tragic' Maine shooting, urges assault weapons ban
US President Joe Biden on Thursday condemned the "senseless and tragic" mass shooting in Maine which killed 18 people and he called on rival Republicans to back a ban on high-powered weapons.
"Today, in the wake of yet another tragedy, I urge Republican lawmakers in Congress to fulfill their duty to protect the American people. Work with us to pass a bill banning assault weapons and high-capacity magazines," the Democratic president said in a statement.