The US Justice Department is investigating whether some of America’s biggest airlines have colluded to keep airfares high, striking at an industry that has posted record profits recently while limiting routes and affordable seats.

Reducing supply and rising demand means increased prices. The antitrust investigation appears to focus on whether airlines illegally signalled to each other how quickly they would add new flights, routes and extra seats.

A letter received by major US carriers demands copies of all communications the airlines had with various parties about their plans for passenger-carrying capacity.

American Airlines, Delta Air Lines, Southwest Airlines and United Airlines all said they received a letter and are complying.

A series of bankruptcies and mega-mergers over the past decade has reduced the number of major US airlines from nine to four – American, Delta, Southwest and United now control more than 80 per cent of the seats in the US domestic market.

They’ve eliminated unprofitable flights, filled more seats on planes and made an effort to slow growth to command higher airfares. It worked. The average domestic airfare rose 13 per cent from 2009 to 2014 and that doesn’t include the billions of dollars airlines collect from new fees.

During the past 12 months, the airlines took in $3.6billion in bag fees and $3billion in reservation-change fees. That has led to record profits.

This year could bring even higher profits thanks to a massive drop in the price of fuel – airlines’ single highest expense. In April, US airlines paid $1.94 a gallon, down 34 per cent from the year before.

The decrease has still not benefited the traveller, however, and it is interesting to see how this investigation develops in the coming months.