Authorities had been monitoring construction of the tunnel for eight months and sealed it shortly after it opened Wednesday, making three to five arrests in the process, a government employee who had been briefed by local law enforcement officials told The Associated Press.
The employee spoke Wednesday on the condition of anonymity because the news had not been made public.
A news release Thursday morning from U.S. Attorney John McKay confirmed the tunnel, calling it the first found on the northern border. A news conference was scheduled for later Thursday, and no further details were given.
Federal authorities have discovered at least 12 tunnels along the U.S.-Mexico border since Sept. 11, 2001, some of them only partially completed. Last year, Homeland Security began using technology developed by geophysicists to pinpoint suspicious terrain.
The U.S.-Canada ran from a greenhouse on the Canadian side to within about 100 yards of an abandoned house on the U.S. side, 300 feet from the border, according to the government source.
Michael Milne, a spokesman for U.S. Immigration and Customs Enforcement, declined to comment. The source said the investigation was handled largely by the Drug Enforcement Administration and the FBI, but FBI agent Roberta A. Burroughs in Seattle said it was "another federal agency's investigation."
The tunnel was found north of Lynden, about 90 miles north of Seattle.