United Airlines Inc. has been fined $2 million by the U.S. Department of Transportation for violating its rules for servicing disabled travelers.

The department investigated the Chicago-based airline's compliance with its regulations implementing the Air Carrier Access Act due to a “significant” increase in the number of disability-related complaints that United received from consumers in 2014, according to a department news release issued last week. A review of these complaints revealed that the airline failed to provide disabled passengers with prompt and adequate assistance in boarding and deplaning aircraft and moving through several airports, including Houston International Airport, Chicago O'Hare International Airport, Denver International Airport, Newark International Airport and Dulles International Airport, according to the release.

In addition, the department's enforcement office found that United failed to return passengers' wheelchairs, other mobility aids or other assistive devices in a timely manner or in the same condition it received them in numerous instances, according to the release.

United will spend $150,000 of the $2 million to improve quality assurance audits of its wheelchair vendors, including tracking the time period for providing wheelchair assistance, while $500,000 will go toward a pilot program to develop and implement technology to assist disabled passengers in making assistance requests via United's mobile app.

The department is also crediting United with $650,000 for compensation it provided to consumers who filed disability-related complaints in 2014, according to the release.

The remaining $700,000 of the assessed penalty is due to the U.S. government's treasury within 30 days, according to the Jan. 7 order.

The airline is also being fined $750,000 for five lengthy tarmac delays exceeding the department's 3-hour limit for domestic flights at Chicago O'Hare in December 2013 and a lengthy tarmac delay of a flight diverted to Houston Hobby Airport in May 2015, according to the release.

“It is our duty to ensure that travelers with disabilities have access to the services they need, and that when significant tarmac delays happen, travelers are not left on the plane,” Transportation Secretary Anthony Foxx said in a statement. “We will make sure that airlines comply with our rules and treat their passengers fairly.”

United receives nearly a million requests for wheelchair assistance every year and is working on making the process of receiving a wheelchair as seamless as possible, including testing the pilot project for the United app, which will allow disabled customers to self-identify, request a wheelchair or other assistance and immediately advise them of the status of their request, according to a statement posted last week on United Hub.

“We expect this to greatly improve our ability to have wheelchairs where they need to be, when they need to be there, so that our customers can get on their way home or to their next destination with ease,” Jon Roitman, United's senior vice president of airport operations, said in the statement.

The airline has also invested $3.8 million in facility improvements such as additional elevators and wheelchair lifts at several major airports and plans to increase that investment to more than $5.3 million in 2016.