The Department of Justice (DOJ) announced Monday that investigators have identified more than 400 noncitizens illegally registered to vote in Washington state, some of whom have cast ballots in multiple elections dating back decades.

Many on the list are green card holders with legal immigration status and permanent residency, according to the U.S. attorney’s offices for the Eastern and Western districts of Washington.

"We have identified over 400 noncitizens who are registered to vote and have voted in multiple elections. This is only the tip of the iceberg," Acting U.S. Attorney Neil Floyd for the Western District of Washington said in a statement. "We will continue to investigate and identify those who are committing crimes by voting illegally." The move builds on the DOJ's legal action against Los Angeles County, where it has sought to set aside roughly 20,000 mail-in ballots sent to individuals whose citizenship status is in question.

Meanwhile, immigration officials told Fox News Digital Tuesday that dozens of noncitizens have also been arrested nationwide in a major voter fraud crackdown.

In an interview with Fox News Digital, Floyd said investigators identified hundreds of noncitizen voters by reviewing U.S. Citizenship and Immigration Services (USCIS) records, which showed that some people who applied for certain benefits had also voted.

While some individuals voted only once or three times in past elections, Floyd said others voted as many as 27 times. In total, his office counted approximately 1,500 instances of noncitizens casting ballots.

He emphasized that some noncitizens had been voting for decades, predating Washington’s transition to a vote-by-mail system.

"I think the earliest moment we have is 1984," Floyd said. "It's early 90s, late 80s, I believe."

However, Floyd said he had not checked whether any of the individuals voted in the upcoming midterms, explaining that he did not want to "interfere with the current election." The number of potential noncitizen voters could represent only a fraction of the broader population, Floyd said, because the list includes only individuals who sought certain benefits in Washington state.

"If you take that population, and you expand it out to include all the people that are here illegally that we haven't had contact with, just my estimation is that it's going to be in the thousands," Floyd said.

Federal officials said many green card holders were registered to vote without their knowledge through Washington’s Motor Voter program, which integrates voter registration into the state’s Department of Licensing.

"We have heard from green card holders who were registered against their knowledge via the Washington Motor Voter program. Then they receive a ballot in the mail and assume they are legally registered — that simply is not the case," the notice said.

According to the Washington secretary of state’s website, eligible residents are automatically registered to vote only if they provide proof of U.S. citizenship during a driver’s license or ID card registration process.

The findings have prompted federal officials to identify patterns in how the voter registration process has failed, Floyd said.

According to the DOJ notice, federal officials are asking Washington state to set aside ballots cast by the identified individuals until their citizenship status can otherwise be confirmed.

"Washington is now on notice that noncitizens are voting. It is up to the state to take action to make sure that they are not allowing these noncitizens to vote illegally," the notice said.

Officials warned that voting as a noncitizen can have serious consequences for those seeking to become U.S. citizens. Potential criminal charges include making false statements in a naturalization application, while some individuals could also face charges for fraudulent voter registration.

On Tuesday, the Department of Homeland Security (DHS) told Fox News Digital that numerous illegal aliens had also admitted to unlawfully registering to vote or casting ballots in U.S. elections. Of the dozens of suspects arrested, at least 18 had voted in a federal election.

The individuals were from a range of countries, including Mexico, Somalia, Honduras, Guyana, Cuba, Germany, Nicaragua, Venezuela, Vietnam, El Salvador, Cambodia, Guatemala, Portugal and Italy.

Many also faced charges for numerous alleged crimes, including counterfeiting, fraud, kidnapping, DUI, sexual offenses, assault with a dangerous weapon, drug possession and burglary.