Federal safety investigators intensified their scrutiny of a Monday fire aboard a Boeing 787 as concerned investors sold shares in the aircraft maker for a second day.

Boeing confirmed Tuesday that the fire aboard a Japan Airlines plane appeared to have started in a battery pack for the plane’s auxiliary power unit while at Boston’s Logan International Airport.

The National Transportation Safety Board described the fire damage to the battery as “severe,” and said it is sending two more investigators to examine the plane. It also formed investigative groups to look at the plane’s electrical systems as well as the fire response.

Meanwhile, a different JAL 787 at Logan had to be towed back to the gate after spilling 40 gallons of jet fuel as it was taxiing toward takeoff around midday Tuesday.

Boeing has a lot riding on the 787, which is assembled in North Charleston and Everett, Wash. The long-range jet promises a smoother travel experience and is 20 percent more fuel efficient than older models.

After years of delays, Boeing has delivered 49 of the planes, including four from North Charleston. With almost 800 more on order, the company plans to double its production rate over the course of this year. The jets involved in this week’s incidents in Boston were made in Everett.

The fire Monday happened on the ground, with no passengers on board. Since in-flight fires can be catastrophic, the matter is getting close scrutiny by aviation authorities.

Interest is especially high in this fire because of indications that lithium batteries, which generally have not been used on large planes before the 787, can burn very hot, with fires that are difficult to put out. The NTSB said it took firefighters 40 minutes to put out the fire near the back of the plane, and that one firefighter sustained minor injuries in the process.

JAL spokeswoman Carol Anderson said the airline inspected its 787s after the fire. She declined to discuss the results, but noted that no schedule changes were made as a result of the inspections.

United Airlines, the only American carrier flying 787s so far, said it inspected its jets overnight. A spokeswoman would not say what the inspections found. The Wall Street Journal reported that the inspections found an improperly installed bundle of wires connected to the same battery that burned on the JAL plane.

Electrical problems have been the 787’s nemesis.

A BB&T Capital Markets analyst lowered his rating of Boeing’s stock from buy to hold in a note titled, “Teething Pains Too Serious to Ignore.”

Shares of Boeing have fallen 4.6 percent since the fire was reported, wiping out almost $2.7 billion of the company’s value. On Tuesday they fell $2, or 2.6 percent, to close at $74.13.

Investors may have been unnerved by the latest incidents after it appeared that the mechanical and manufacturing kinks with the 787 had been worked out.

Brendan Kearney of The Post and Courier contributed to this report.