Three law enforcement officials involved in the case told Sky's US partner site NBC News officers had "no hard leads" as to the whereabouts of Robert Card, 40.

There have been no confirmed sightings of him either since Wednesday night's attack, which was the deadliest mass shooting in the US this year.

It came as officials held another press conference on Friday night in which they confirmed the names of all 18 victims.

Public safety commissioner Mike Sauschuck also announced that a "shelter in place order" for residents in the region had been rescinded. However he said hunting would be prohibited in the areas surrounding Lewiston, Lisbon, Bowdoin and Yarmouth.

Mr Sauschuck also confirmed officers were investigating a note apparently left by Card, but would not comment on its contents.

Police are examining whether it may have been a suicide note, sources have told US media.

Mr Sauschuck said a search of the River Androscoggin, near where the suspected gunman's abandoned car was discovered, would continue.

"We are not going to finish that search this evening," he told reporters on Friday.

At an earlier briefing, Mr Sauschuck revealed police were investigating more than 500 tips and leads, and said officers were working round the clock through them.

Maine shooting latest: Divers searching river near where suspect's car found

Hundreds of police officers and FBI agents have been searching for the suspect.

Card is believed to have killed seven people at Just-In-Time Recreation bowling alley, and a further eight people at Schemengees Bar and Grille in Lewiston.

Three more people died after being taken to local hospitals.

Among the dead are a father and son, a volunteer bowling coach and a manager at the bar, which the suspect is reported to have visited before.

Armed police and a helicopter surrounded a home in Bowdoin, near Lewiston, for more than two hours on Thursday night and executed a search warrant before leaving.

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'Armed and dangerous' suspect still on run
What do we know about the Maine shooting suspect?

Who is the suspect?

Card is thought to have 20 years of military service, previously worked as an army firearms instructor and recently spent time at a mental health facility after hearing voices, according to an internal police notice.

He is said to have been a sergeant first class in the army reserve, specialising in petroleum supply, but had no previous combat deployments overseas since enlisting in 2002, according to the US Army.

During his time in the forces, he was awarded the Army Achievement Medal, Army Reserve Component Achievement Medal, Humanitarian Service Medal, National Defence Service Medal and Army Service Ribbon.

'Horrific tragedy'

Just-In-Time Recreation bowling alley posted an emotional Facebook message in the aftermath of the attack.

"None of this seems real, but unfortunately it is," it read. "We are devastated for our community and our staff. We lost some amazing and wholehearted people from our bowling family and community last night.

"There are no words to fix this or make it better. We [are] praying for everyone who has been affected by this horrific tragedy. We love you all and hold you close in our hearts."

Schemengees Bar and Grille also posted a message on the social media platform.

"My heart is crushed," the post said. "I am at a loss for words."

The unidentified writer said their world had been turned "upside down", and referred to the "loss" of lives.

"How can we make any sense of this. Sending out prayers to everyone," they added.