Honeywell said Monday that it has joined the investigation into last week's fire on a 787 at London's Heathrow Airport amid reports that investigators are eyeing whether its emergency transmitter played a role in the incident.

The emergency location transmitter sends a signal with the plane's location if it's involved in a crash. It's meant to help rescuers find the plane.

The fire on a parked Ethiopian Airlines 787 Friday brought back memories of two smoldering lithium-ion batteries in January that caused authorities to ground the 787 worldwide for more than three months. Boeing redesigned the systems that include those large, powerful batteries to win government approval so 787s could fly again.

The emergency location transmitter uses a lithium-manganese battery. That is a type of lithium-ion battery, but with different chemistry that is considered more stable than the batteries that were an issue in the grounding.

The U.K.'s Air Accidents Investigation Branch said on Saturday that the most recent fire occurred well away from the lithium-ion batteries. Published reports Monday said investigators were examining whether the transmitter could have caused or contributed to the fire.

A spokeswoman on Monday declined to say whether the focus had shifted to the emergency locater, and warned against speculation on the causes of the incident. "The traveling public can be sure we're investigating all possible causes and following up all leads," she said.

Honeywell declined to specify why authorities asked it to participate in the investigation.

"It's far too premature to speculate on the cause, or draw conclusions," spokesman Steve Brecken said in a written statement. He directed other questions to the U.S. National Transportation Safety Board, which is helping with the investigation in the U.K.

Brecken said Honeywell's emergency transmitters "have been certified by the FAA since 2005, are used on a number of aircraft models, and we've not seen nor experienced a single reported issue on this product-line."

United Airlines is the only U.S. airline to fly the 787. It said it has maintained its 787 flight schedule. Ethiopian Airlines has also said it is continuing to operate its 787s.

Boeing shares rose $3.79, or 3.7 percent, to close at $105.66. Investors were relieved that the fire doesn't appear to involve the lithium-ion batteries. Shares of Honeywell International Inc. fell 7 cents to close at $82.30.