Southwest Airlines said it would launch an investigation into whether or not some of its mechanics conspired to cause flight disruptions in retaliation for the airline’s refusal to meet certain demands during mediated contract negotiations. In a statement issued late Tuesday, Southwest Airlines chief operating officer Mike Van de Ven reported that on February 12 the airline suffered what he called an unprecedented number of airplanes pulled from service in four of 20 maintenance locations despite no change in maintenance programs, leadership, or policies and procedures.

Southwest has removed from service more than 40 of its fleet of some 750 Boeing 737s since February 12, or about double the number that usually undergo maintenance at one time. The airline canceled 191 flights on Tuesday and attributed roughly half to maintenance issues. It also experienced 858 delays, the largest number in the country.

The airline said it would assign as much scheduled maintenance work as possible to third-party vendors in an effort to mitigate the disruptions.

“We are committed to operating a safe fleet, and every report is investigated, which is why we issued a notice to require an ‘all hands’ response to get out-of-service aircraft back into the fleet serving our customers,” said Van de Ven.

Southwest and the Aircraft Mechanics Fraternal Association (AMFA) have negotiated for more than six years on a new contract. The airline said it offered its 2,400 mechanics “an industry-leading pay package” last autumn, which they rejected. Since then, said Van de Ven, the airline has “enhanced” its previous offer.

“AMFA has a history of work disruptions, and Southwest has two pending lawsuits against the union,” he explained. “We will be investigating this current disruption and exploring all possible remedies.”