The Trump administration accused the state of California Thursday of “failing to meet its obligations” to protect the environment, claiming that a growing homeless population threatens the state’s water quality.

Days after President Trump mocked Los Angeles and other big cities for their “tremendous pollution,” EPA Administrator Andrew Wheeler sent an oversight letter charging California officials with failing to meet federal health standards in numerous communities where large homeless populations litter the streets with trash, drug paraphernalia and human waste.

The unusual move ratchets up the Trump administration’s ongoing assault on the nation’s most populous state, a multipronged campaign that California Gov. Gavin Newsom (D) has described as “weaponizing” the federal government. The outcome of that battle — which has involved the Justice Department and the Department of Homeland Security, as well as the EPA — could determine who gets to set policy on a range of issues, from immigration to transportation.