Hundreds of police were deployed across Maine looking for the suspect, who is on the run after shooting dead 18 people in the city of Lewiston.
Police advised people in the region to stay indoors and schools across the area remained closed on Thursday. State and local police identified the suspect as Robert R. Card, who reportedly had been committed to a mental health facility over the summer.
"We have literally hundreds of police officers working around the state of Maine to investigate this case to locate Mr. Card, who is a person of interest," Maine Public Safety Commissioner Mike Sauschuck told a press conference.
Meanwhile, Maine governor said Card is considered to be armed and dangerous. "The suspect is still at large. Robert Card is considered armed and dangerous. This is a dark day for Maine...has deployed all available state resources including mental health professionals...18 people killed in Maine shootings and 13 people were wounded," said Maine Governor Janet Mills.
Earlier, reports said 22 people were killed in the shooting incident.
Police also found a white SUV they believe Card drove to the town of Lisbon, about 7 miles (11 km) to the southeast. Sauschuck added that people were asked to remain indoors in both Lewiston and Lisbon as the manhunt continues.
Maine police also told residents in the town of Bowdoin, about 12 miles east of Lewiston, to shelter in place.
Lewiston is a former textile hub and town of 38,000 people in Androscoggin County about 35 miles (56 km) north of Maine's largest city, Portland
US President Joe Biden spoke with the officials and offered the federal government's support, his office said.
Authorities in north-eastern states close to Maine are on alert as well. New Hampshire's homeland security agency said they were monitoring the situation. The New Hampshire-Maine state border is around 50 miles from Lewiston.
Colonel Stavros Mellekas, deputy commissioner of Connecticut's State Police called the incident "deeply disturbing and heart-breaking".
"At this time, our focus will be on protecting communities within our state," read a statement by Mellekas.
