The desperate manhunt for suspected Maine mass shooter Robert Card has extended to Canada, with authorities issuing an alert for the “armed and dangerous” trained marksman — as it was revealed his online search history contained conspiracy theories, including about President Biden.

The Canada Border Services Agency said it issued the alert to officers along the US-Canada border to “ensure the safety and security of Canadians and protect Canada’s borders against any threat or [attempted] legal entry.”

It came three days after 18 people were killed and more than a dozen others injured in a Wednesday night shooting spree at multiple sites in Lewiston, Maine.

The mayor of a border town in Canada also said he was aware of American authorities at the border “checking people leaving the country” and said Canadian police are “doing the same as we speak,” according to Global News.

St. Stephen Mayor Allan MacEachern put out his own alert to remind residents to be vigilant.

“We’re going to make sure everyone knows: Keep your eyes and ears open for any strange activity,” he told Global News.

The Canadian alerts came as in the US, more than 350 state and federal law enforcement officials continued the manhunt for Card, thought to be the gunman caught on surveillance footage during the massacre of 18 people in two different locations in Lewiston, Maine, late Wednesday.

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The Coast Guard announced Thursday it dispatched a boat crew from Boothbay Harbor and a wing plane from Cape Cod to assist in the search efforts.

Card, a skilled outdoorsman and trained US Army Reserve marksman, owns a 12-foot Sea-Doo jet ski registered in his name and had previously registered a 15-foot bay liner boat, according to documents obtained by CBS News.

But after hours of searching, authorities found “nothing out of the ordinary,” Chief Petty Officer Ryan Smith, who is in charge of the Coast Guard’s Boothbay Harbor Station, told KATV.

Rick Goddard, a longtime neighbor of Card’s, told The Post the suspect is “capable of hiding for a long time if he doesn’t want to be caught.”

“It’s his stomping ground,” Goddard said of Card’s hometown of Bowdoin.

“I mean, he grew up here. He knows the area really well. He’s capable of hiding for a long time if he doesn’t want to be caught. There’s a lot of places you can be in the woods and never be seen.”

“It’s really dense, thick woods. If you know the area, there’s a lot of places you can hide … you could never see something like that from an airplane or helicopter.

“The trees are so close together, you can barely walk through them. It’s so thick, you can barely see 10 feet into the woods. If I was going to hide, that’s what I would do. I would hide in some place like that.”

Meanwhile, authorities revealed that Card appears to have “interacted with conspiratorial content” online, ABC News reported.

Topics he engaged with online included concerns about a financial crisis, LGBTQ+ issues, gun rights, and commentary about President Biden and other Democratic lawmakers, the outlet said.

Police in Maine said Card walked into the Just-In-Time Recreation Bowl in Lewiston shortly before 7 p.m. Wednesday and opened fire with an assault rifle, killing seven people.

Minutes later, he struck at Schemengees Bar & Grille about two miles away, killing seven more inside the establishment and one outside, cops said.

Three others who were wounded at the two sites were later pronounced dead at area hospitals, while an additional 13 people were injured by gunfire — including a 10-year-old girl.

“We believe this is someone who should not be approached,” Maine State Police Col. William Ross warned at a press briefing Wednesday.