A group of high-profile Democratic Senators, led by Sherrod Brown of Ohio, wrote letters to the nation's top retail CEOs Monday, demanding they crack down on trucking companies that turned their workers into modern-day indentured servants.

The call to action comes in response to a yearlong USA TODAY Network investigation that found port trucking companies in California forced their drivers into debt, pressured them to work up to 20 hours a day and paid them pennies per hour.

"As a major U.S. corporation," the senators wrote, "you also have a role to play in ensuring that you are not complicit in the mistreatment of port truck drivers and that American consumers, your customers, are not unwittingly supporting labor abuses in the United States."

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Sen. Elizabeth Warren, D-Mass., and Sens Dianne Feinstein and Kamala Harris of California joined Brown in sending the letter to 16 retail brands, including Target, Wal-Mart and Costco. They asked each CEO to answer a series of questions about their knowledge of labor violations in the port trucking industry and about their plans to cut ties with those companies.

Together, these four senators represent powerful voices around the issues of labor, immigration and corporate malfeasance.

Warren, of Massachusetts, famously took on Wall Street after the real estate collapse in 2008 and transformed her popularity as a consumer crusader into a Senate bid. California Sen. Feinstein is the ranking Democrat on the powerful judiciary committee.

"Port trucking companies' brazen disregard for federal transportation safety standards and workers' safety and rights is shameful," they wrote in Monday's letter.

They pledged to "pursue aggressively all federal avenues to put an end to this rampant mistreatment of port truck drivers."

"We cannot allow corporate profits to come at the expense of American workers, and that means putting an end to abusive practices we've seen in the port trucking industry," Brown said in a statement.

Since USA TODAY Network published its investigation in June, elected officials across the country have been pressing for action.

Dozens of lawmakers, from the Los Angeles City Council to Capitol Hill, have denounced the business practices, many calling on retail corporations to better police their trucking contractors.

Los Angeles City Councilman Joe Buscaino wrote a letter to the CEO of Target, suggesting the retail giant adopt a code of conduct specific to its port trucking contractors.

In a follow-up statement to reporters, Buscaino said he wants a "commitment from the big box retailers to stop doing business with trucking companies that treat their drivers as indentured servants."

Target, which previously called the mistreatment of workers in its supply chain "unacceptable," declined to comment about the letters. Others, contacted Monday afternoon by USA TODAY, did not have an immediate response.

As they investigated the port trucking industry, reporters contacted two dozen retail companies, including Target, Costco and LG, that have used port trucking companies with labor violations. The few that responded said they were either unaware of widespread labor violations or not responsible for monitoring the maze of contractors and subcontractors in their shipping operation.

Now, some companies say they are taking steps to identify any abusive companies in their supply chain.

"We're investigating this issue directly with carriers," Home Depot spokesman Stephen Holmes said. Holmes pointed out that Home Depot had previously changed trucking companies and no longer uses any companies named in the USA TODAY investigation.

Over the past month, elected officials have met with experts and labor union leaders to discuss new laws that could curb abuse in the industry. And they've begun publicly pressuring federal Department of Transportation regulators to better coordinate with state officials to ferret out predatory labor practices.

Rep. Grace Napolitano, D-Calif., said she's working with Rep. DeFazio, D-Ore., and Rep. Jerrold Nadler, D-N.Y., to resurrect a federal bill, the Clean Ports Act, which would give states and cities the power to better enforce labor laws and standards in the port trucking industry.

In California, state Rep. Ricardo Lara said he's drafting another bill to tweak California law so companies would have to disclose how they audit their operations at American ports.

California law already requires big companies doing business there to disclose what efforts they make to monitor their supply chain overseas. But retailers are not required to apply those standards to their U.S. shipping operations.

"We can't ignore the reality of what's happening in our backyard," Lara said. "I think it's one of the biggest injustices happening in our state."

Federal lawmakers have also called on regulators to begin working with state officials to coordinate efforts to crack down on labor law violators.

Two weeks ago, Rep. Alan Lowenthal, D-Calif, and Rep. DeFazio quizzed Department of Transportation officials on whether they had begun taking action at the ports.

Unhappy with the response, Lowenthal said he may seek to compel federal and state coordination if the agencies don't quickly take steps on their own.

DOT officials said the agency typically investigates cases only after they receive complaints.

Hundreds of California drivers have testified in state labor cases that they were cheated out of wages. Many described potential crimes, including managers who coerced them to work past the legal limit of hours and doctor logs, at times physically preventing workers from going home.

But that information has not yet led to criminal investigations at the federal or state level.

Julie Su, the California Labor Commissioner, told reporters in December that her office has not passed along port truckers' testimony to the DOT or the California Highway Patrol.

"I don't have a good answer to that," Su said when asked why the agency did not sharethe evidence. "It hasn't been something we pursue."

The USA TODAY Network's investigation focused on trucking companies around Los Angeles, where two ports are responsible for about half of all imports brought into the country.

In 2008, California officials ordered trucking companies at the ports to replace old big rigs with cleaner trucks. The companies pushed that cost onto drivers with a lease-to-own program, forcing many drivers to work around the clock. When they got sick or fell behind on payments, companies fired them, seizing their trucks and thousands they had paid in.

Company owners deny the claims made by their drivers, arguing most drivers are successful, and those who aren't have the freedom to quit.

Weston LaBar, executive director of the Harbor Trucking Association, which represents port trucking companies, said most of the complaints so far have been "noise drummed up by the Teamsters union."

"If policymakers feel inclined to investigate this issue further," LaBar said in a later email, "hopefully they will reach out to industry as well to get all perspectives."