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Boeing said Wednesday it has "extreme confidence" in its 787 Dreamliner, even as federal investigators try to determine the cause of a fire that along with other issues this week has prompted new worries about the plane.

The fire occurred Monday in one of the plane's lithium ion batteries. Mike Sinnett, Boeing's chief engineer for 787, wouldn't comment on that specific incident, but told reporters that the battery is designed to avoid overheating and the area around the battery is designed to withstand a fire.

But questions remain about the high-profile jet, which has a lot riding on it both for Boeing and its airline customers. After a nearly three-year delay, Boeing has delivered 49 of the 787s so far, and has about 800 more on order. The company has a plant in Salt Lake City that makes the horizontal tail fin stabilizers used on its 787s.

Investors rallied behind Boeing Wednesday, with shares gaining 3.2 percent, to $76.47, after dropping 4.6 percent the two previous days.

The Dreamliner has had a rough stretch. Besides Monday's fire aboard, which is being probed by federal safety officials, an empty Japan Airlines plane in Boston, a separate JAL 787 experienced a fuel leak on Tuesday. And All Nippon Airways canceled a domestic 787 flight in Japan Wednesday when a computer system indicated a problem with the plane's brakes. Last month, a United Airlines 787 flying from Houston to Newark, N.J., diverted to New Orleans because of an electrical problem with a power distribution panel.

Sinnett says the problems Boeing has seen so far with the 787 are similar to early issues with the Boeing 777, which was introduced in the mid-1990s.

The battery fire is of particular interest because lithium batteries generally have not been used on large planes before the 787. Sinnett says the nature of lithium ion batteries means no fire extinguisher system will stop them from burning once they start. The National Transportation Safety Board said it took firefighters 40 minutes to put out Monday's fire.