ST. IGNACE, Mich. (AP) - A federal agency is trying to allay concerns of three U.S. senators who asked for evidence that an oil pipeline crossing a section of the Great Lakes is safe. Cynthia Quarterman, the head of the federal Pipeline and Hazardous Materials Safety Administration, wrote U.S. Sen. Debbie Stabenow recently to say "significant safety improvements" have been made. The request from Stabenow and Carl Levin of Michigan along with Richard Durbin of Illinois came in December. Enbridge Energy Partners LP last year boosted capacity of its Line 5 pipe roughly 10 percent. It runs from Superior, Wis., to Sarnia, Ontario, passing through Michigan's Upper Peninsula and crossing the Straits of Mackinac. Another Enbridge pipeline ruptured in southern Michigan in 2010, spilling about 843,000 gallons of oil.