Federal agents have shut down a drug-smuggling tunnel built under the U.S.-Canadian border near Lynden, Wash., a federal source told The Associated Press on Wednesday.

Authorities had been monitoring construction of the tunnel for eight months and sealed it Wednesday, shortly after it opened, making three to five arrests in the process, said the source, a government employee who had been briefed by local law enforcement officials.

The exact length of the tunnel was not known. It ran from a building on the Canadian side to a house on the U.S. side, 300 feet from the border, the source said.

The source spoke on the condition of anonymity because the news had not yet been made public; a press release was expected Thursday morning.

Emily Langlie, a spokeswoman for the U.S. Attorney's Office (search) in Seattle, declined to comment, as did Michael Milne, a spokesman for U.S. Immigration and Customs Enforcement (search).

The source said the investigation was handled largely by the Drug Enforcement Administration (search) and the FBI (search), which did not immediately return a call seeking comment Wednesday.