Top House Democrats are asking the FBI to investigate whether connections between Donald Trump’s campaign aides and Russian interests led to the cyberattacks at the Democratic National Committee and Democratic Congressional Campaign Committee.

“We are writing to request that the FBI assess whether connections between Trump campaign officials and Russian interests may have contributed to these attacks in order to interfere with the U.S. presidential election,” the lawmakers wrote in a letter sent to FBI Director James Comey on Tuesday.

The letter was signed by Rep. Elijah Cummings, D-Maryland, ranking member on the House Oversight and Government Reform Committee; Rep. John Conyers, D-Michigan, ranking member on the House Judiciary Committee; Rep. Eliot Engel, D-New York, ranking member on the House Foreign Affairs Committee; and Rep. Bennie Thompson, D-Mississippi, ranking member on the Homeland Security Committee.

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The lawmakers said that “overt and covert actions by Trump campaign officials on behalf of Russian interests” have raised “serious questions.”

They referenced several reports about Trump campaign officials’ or his advisers’ ties to Russia. They mentioned long-time Trump adviser Roger Stone’s admission in early August that he had previously communicated with WikiLeaks founder Julian Assange. They also said Trump foreign policy adviser Carter Page traveled to Moscow and delivered a speech critical of the U.S. and how Lt. Gen. Michael Flynn, who’s endorsed Trump, traveled to Moscow last December and ate at Russian President Vladimir Putin’s table.

They also expressed concern about Paul Manafort, who served as Trump’s campaign chairman until he resigned recently after a series of damning reports about his involvement in Ukraine.

In the letter to Comey, they wrote that they were also concerned when Trump appeared to encourage Russia to track down 30,000 personal emails that Hillary Clinton deleted from her tenure as secretary of state.

“We do not know if Donald Trump’s public statements or the connections of his campaign officials to Russian interests directly or indirectly led to the cyber attacks against Democratic party organizations, but there is widespread agreement that the United States should take all possible steps to prevent Russia from interfering in our electoral process and prosecute to the full extent of the law anyone involved in such a scheme,” the letter said.

A request for comment by the Trump campaign was not immediately answered.

The FBI is already investigating the hacking of the DNC and DCCC that involved Wikileaks disclosing more than 19,000 internal DNC emails in July. U.S. officials have said they believe people working for the Russian government were behind at least the DNC attack.

The Democrats’ request comes a day after CBS News confirmed that the FBI found that hackers accessed Arizona’s and Illinois’s state election databases. Senate Minority Leader Harry Reid, D-Nevada, is asking the FBI to look into the possibility that Russia is trying to manipulate the U.S. presidential election.