WASHINGTON (Reuters) - The U.S. National Transportation Safety Board said on Friday it had opened an investigation into the Colonial Pipeline Co accident in Helena, Alabama, which killed one worker and seriously injured four others.

Colonial's gasoline pipeline ruptured on Monday during routine maintenance operations. The company said Friday it expects to restart the pipeline on Sunday afternoon.

A team of five NTSB investigators arrived Thursday and will be in Alabama for several days conducting interviews, documenting the accident site and collecting evidence. Investigators will also visit the Colonial Pipeline offices in Alpharetta, Georgia, to interview operations and engineering staff and collect operating data and documents, the board said.

The 5,500-mile (8,850-km) Colonial Pipeline is the largest U.S. refined products pipeline system and can carry more than 3 million barrels of gasoline, diesel and jet fuel between the U.S. Gulf Coast and the New York Harbor area.

The company said a small fire remaining at the site was extinguished overnight, and operations to remove residual gasoline from the pipeline began earlier on Friday.

(Reporting by David Shepardson; editing by Tom Brown, G Crosse)