(Reuters) -The largest firefighters' labor union in the U.S. is demanding antitrust authorities investigate the companies that make fire trucks, saying industry consolidation has led to skyrocketing costs and years-long wait times, endangering fire fighters and the public.

The U.S. Department of Justice and Federal Trade Commission should probe the fire and emergency vehicle industry, where three companies - REV Group, Oshkosh, and Rosenbauer - together make up around two-thirds of the market, the International Association of Fire Fighters said in a letter.

Truck prices have doubled in the past decade, with ladder trucks now costing as much as $2 million each, while fire departments are facing backlogs as long as four years, said IAFF, which represents fire and emergency workers in the U.S. and Canada, and American Economic Liberties Project, an antimonopoly group.

"It's really a critical hazard in public safety," IAFF President Edward Kelly told Reuters.

Oshkosh spokesperson Lisa Barwick said the company is investing in manufacturing capacity, technology, and process improvements to meet demand.

"Global supply challenges, unprecedented demand, and significant inflation since the pandemic started in 2020 have resulted in extended delivery times and increased prices," she said.

A REV Group spokesperson also cited demand, labor shortages and cost increases as driving price increases and delays.

"In response, we have increased production of our fire and emergency vehicles by nearly 30% in the last two years, introduced new lines of semi-custom trucks with faster ship times, and continued to invest in training programs for skilled talent," the spokesperson said.

Rosenbauer did not immediately respond to a request for comment.

IAFF and AELP said high prices and long waits are endangering public safety in communities facing natural disasters, citing reports that dozens of fire trucks were out of commission during the wildfires that devastated two Los Angeles communities and killed at least 29 people earlier this year.

From Atlanta, to Houston, to San Francisco, cities and towns are facing a crisis where demand for new fire trucks outstrips availability and funding, Kelly said.

On top of long wait times, fire departments are being pushed into contracts with "floating" pricing structures, where the final price of a truck may go up after an order is placed.

"We are paying the price for all these corporate decisions. It serves the investor well, but it doesn't serve the public when you call 911 and the ladder truck is out of service," Kelly said.

The DOJ or FTC should use their authority to investigate the industry, and the FTC should launch a study of how consolidation is affecting fire departments' access to a broader range of parts and services, they said.

Fire departments are seeing the effects of consolidation on everything from dispatch software to personal protective equipment and vehicle parts, Kelly said.