United Airlines is facing millions in federal fines for failing to properly accommodate travelers with disabilities on numerous occasions.

The U.S. Department of Transportation said this week that it is fining the airline $2 million in response to disability-related complaints in 2014.

An investigation found that United did not provide proper assistance for passengers with disabilities getting on and off planes and in moving through airport terminals in Houston, Chicago, Denver, Newark, N.J. and at Washington Dulles International Airport, the Transportation Department said.

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What’s more, the federal agency said that the airline often failed to return wheelchairs and other mobility or assistive devices to passengers in a timely fashion or in the same condition which they started.

“It is our duty to ensure that travelers with disabilities have access to the services they need,” said U.S. Transportation Secretary Anthony Foxx. “We will make sure that airlines comply with our rules and treat their passengers fairly.”

A portion of the $2 million fine will go toward improving United’s services for people with disabilities. The airline will spend $150,000 to enhance quality-assurance and tracking of vendors who provide wheelchair assistance at airports. Another $500,000 will go toward a pilot program to allow wheelchair and other assistance requests to be made through United’s mobile app.

The Transportation Department is also crediting United $650,000 for payments it already made to consumers who filed disability-related complaints with the airline in 2014.

In a blog post, United said it receives nearly a million requests for wheelchair assistance each year and is working to improve its facilities and processes for accommodating customers with disabilities.

The company said it works with partners to offer opportunities for individuals with autism and other special needs to participate in simulated travel experiences and also relies on an advisory board comprised of individuals with disabilities to help develop its policies.

“We’ve heard from many of you that you expect more from us when serving customers with disabilities,” the company said. “And while we’ve made significant efforts to improve, we remain focused on doing​​ better​.”