Regulators in Bismarck, North Dakota, voted Friday to order a formal investigation into an oil spill from Calgary-based TransCanada Corp.'s Keystone pipeline in the state on May 7.

The pipeline leaked 500 barrels of oil in southeastern North Dakota after a valve failed at a pumping station.

The decision by the North Dakota Public Service Commission will allow it to take testimony about the accident and allow for direct participation by interested members of the public.

Public Service Commissioner Kevin Cramer said the cleanup is going well. But Kramer said he was troubled that the pumping station has had problems so soon.

Keystone, which moves 450,000 barrels of oil a day from Alberta to Illinois and Oklahoma, has been operating for about a year. It is expected to resume operation on Saturday.

Environmentalists say the spill underscores safety concerns about TransCanada's proposal to extend the Keystone pipeline to carry more oilsands crude into the United States.

With files from The Associated Press