U.S. airline stocks were active in early Monday trading as Boeing (BA) - Get Report tumbled more than 12% at the opening bell after the second fatal crash in the past six months of its 737 Max 8 jet Sunday in Ethiopia raised serious safety questions for the flagship program.

Ethiopian Airline flights 302, travelling from Addis Ababa to Nairobi, crashed shortly after takeoff Sunday, killing all 157 passengers on board. The cause of the disaster, which followed a deadly crash of a Lion Air flight off the coast of Indonesia in late October, has not yet been determined, but control tower recordings suggest the pilot was having early difficulties and eye witnesses reported seeing flames from the jet prior to its crash.

"Boeing is deeply saddened to learn of the passing of the passengers and crew on Ethiopian Airlines Flight 302, a 737 MAX 8 airplane," the company said in a statement. "We extend our heartfelt sympathies to the families and loved ones of the passengers and crew on board and stand ready to support the Ethiopian Airlines team."

"A Boeing technical team will be travelling to the crash site to provide technical assistance under the direction of the Ethiopia Accident Investigation Bureau and U.S. National Transportation Safety Board," the statement added.

Boeing shares, which had fallen as much as 12% in early Wall Street trading, were marked 5% lower by mid-afternoon at $401.91 each, a move that clipped more than 140 points from the Dow Jones Industrial Average.

The S&P 500 Airlines industry actually managed to gain around 0.65% by mid-afternoon to trade at 310.72 points, trimming its five-day decline to under 3%.

Delta Airlines (DAL) - Get Report was the biggest ancillary mover on the Boeing story, with shares rising 3% to $51.05 each, while Southwest Airlines (LUV) - Get Report , the largest domestic user of the 737 Max 8, was marked 0.49% lower at $51.52 each.

American Airlines Group (AAL) - Get Report , which owns 24 Max 8 Jets, was seen 0.33% higher at $32.02 each.  United Airlines (UAL) - Get Report , which confirmed via a Twitter statement Monday that it does not use the Max 8, was marked 0.51% lower at $81.98 each.

Another stock on the move with a potential linkage to the Flight ET302 disaster was Marsh & McLennan Companies Inc. (MMC) - Get Report , which swung in and out of positive territory during an active morning session after Reuters reported it is the insurance broker for Boeing, before rising 1.15% to 90.58 by mid-afternoon.

Boeing's 737 lineup, perhaps its most important program, contributes around a third of the the Chicago-based planemaker's bottom line and has a back order log of nearly 5,000 units.

China and Indonesia said they would ground their Max 8 fleets in the wake of the tragedy, while South Korea said it would launch and investigation into any safety issues linked to the Boeing plane.