A United Airlines flight was forced to make an emergency landing at Vancouver International Airport early Saturday morning after a passenger left a threatening message.

Flight 1104 left Anchorage at 11:39 p.m Friday enroute to Denver with 131 passengers and six crew members.

Midway through the flight the crew elected to turn back to Vancouver after finding the message, according to United spokesman Charlie Hobart.

It’s not known what the message said.

Emergency crews surrounded the plane when it landed in Vancouver at 4:15 a.m. Saturday.

A passenger who was on the plane told CTV News the flight landed without any announcement from the pilot or crew.

“They started getting lower midway through the flight and we noticed there was a lot of water... so we weren't near Denver,” said Mike Hatcher, noting that the passenger who allegedly made the threat was removed by police. “A lot of cops started pulling up…drew their guns…[and] came aboard.”

Richmond RCMP confirmed they have a suspect in custody, adding that all passengers were escorted from the flight and security screened.

The witness said the passenger seemed unstable and had been getting up and down a lot in his seat.

“It was a little scary...but I knew they were just doing their job trying to make us all safe,” said Sabrina Gaudet.

United said it’s working to book passengers on alternate flights.

An airport spokesperson said the incident had no effect on airport operations.

With files from CTV Vancouver’s Scott Hurst