The Trump administration last month accused the State of California of violating clean water laws.

Hundreds of former US Environmental Protection Agency employees on Thursday urged a congressional probe into whether the agency’s feud with California represented retaliation for the state’s failure to support President Donald Trump‘s political agenda.

Last month, the Trump administration escalated its fight with California with a letter to Governor Gavin Newsom accusing the state of violating clean water laws by failing to properly prevent human waste from homeless people from entering waterways.

“No evidence supports targeting the homeless for Clean Water Act enforcement, given the much greater damage done by the huge volume of waste discharged from industrial plants, factory farms, and sewage treatment plants and many other sources,” wrote the former EPA employees, including Elizabeth Southerland, a former agency director of science and technology.

“Considering its enthusiasm for de-regulation and reluctance to enforce the laws still on the books, President Trump’s threat to wield EPA’s authority against the most vulnerable members of our society is shameless and morally repugnant,” they said.

The letter followed a call on September 27 by US Senator Dianne Feinstein, a California Democrat, for the EPA’s inspector general to investigate whether politics instead of pollution was behind the Trump administration’s threat to withhold more than four billion dollars in federal transportation funding from California over claims of poor air quality.

The letter, signed by nearly 600 EPA former employees, to US House of Representatives oversight and energy committees also came as a response to Trump’s claim last month that homeless people are responsible for “tremendous” amounts of ocean pollution in California.

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Spearheaded by the nonprofit Environmental Integrity Project, the letter noted that six states have had more large pollution sources in noncompliance with federal environmental laws over the last three years than California: Ohio, New York, Iowa, Missouri, Texas and Indiana. The letter cited agency records.

California has waged numerous policy battles and legal challenges with the Republican president’s administration over environmental protection, immigration, housing and the state’s right to set stricter standards on carbon emissions from vehicles than federal rules.

Michael Abboud, an EPA spokesman, said there was no abuse of authority. “California’s inability to comply with the Safe Drinking Water Act has been an ongoing challenge, and the failure to properly operate and maintain the City’s sewage collection and treatment facilities creates public health risks,” he said.

“Highlighting that California has the worst air quality in the nation along with other serious environmental problems is not a political issue,” Abboud added.