CANNON BALL, N.D. — Construction of the Dakota Access pipeline under a North Dakota reservoir has begun and the full pipeline should be operational within three months, the developer of the long-delayed project said today, even as American Indian activists vowed to fight in the courts to protect their water supply.

The Army granted Energy Transfer Partners formal permission Wednesday to lay pipe under Lake Oahe, clearing the way for completion of the 1,200-mile, $3.8 billion pipeline. ETP spokeswoman Vicki Granado confirmed early today that construction resumed “immediately after receiving the easement.”

Workers had already drilled entry and exit holes for the crossing, and oil had been put in the pipeline leading up to the lake in anticipation of finishing the project.