Washington (CNN) Two men tied to Rudy Giuliani's effort to solicit Ukrainian interference in the 2020 election have been arrested and charged with unrelated campaign finance violations.

The men, Lev Parnas and Igor Fruman, were already in the spotlight as House Democrats ramp up their inquiries and prepare for articles of impeachment against President Donald Trump.

Now they'll face legal problems of their own. They were named in a criminal indictment , unsealed Thursday morning, along with two others who allegedly participated in the scheme.

Here's a breakdown of what we know about these four men and their alleged crimes:

Lev Parnas

Parnas, 47, wears many hats. He's a Ukrainian-American businessman. He's a political donor who cut large checks to pro-Trump super PACs. And he has acted as a personal fixer of sorts for Giuliani, connecting the former New York City mayor to government officials in Ukraine.

To House Democrats, Parnas is a critical witness for their impeachment inquiry. They see him as a participant in the Giuliani-led scheme to get dirt on former Vice President Joe Biden from Ukraine. Parnas has refused requests from Democrats for documents and testimony, which triggered Democrats to issue a subpoena Thursday.

According to the indictment, it was Parnas who allegedly tried to use his influence as a donor to secure the removal of the US ambassador to Ukraine. He did this by meeting with a US congressman "at the request of one or more Ukrainian government officials," prosecutors said.

The lawmaker is not named in the indictment but has been identified by CNN as former Texas Rep. Pete Sessions, a Republican. The ambassador, Marie Yovanovitch, was recalled from her post earlier this year, potentially for political purposes, as detailed in a whistleblower complaint

Parnas was charged with four federal crimes, including violating campaign finance laws, lying to the Federal Election Commission and conspiracy.

Igor Fruman

Fruman, 53, is a naturalized American citizen who was born in Belarus. He was one of Parnas' business partners and also was involved in large political donations to pro-Trump efforts and other Republicans. Prosecutors say some of those funds illegally came from a foreign national.

Using money from a Russian donor, Fruman contributed $10,000 each to two candidates in Nevada during the 2018 cycle, where he and his associates sought a license for their planned marijuana business, according to the indictment.

State records indicate that the donations went to Adam Laxalt, the then-Nevada attorney general and 2018 Republican gubernatorial candidate, and Wesley Karl Duncan, who ran unsuccessfully for attorney general. In response to questions about Fruman, a spokesperson for Laxalt said, "He came to a Las Vegas fundraiser. Adam doesn't know the man."

Additionally, Fruman is linked to Giuliani and the effort to pressure Ukraine to investigate Biden. That effort, which included Trump, is now at the center of the impeachment inquiry.

Like Parnas, Fruman has refused requests from Democrats for documents and testimony, so Democrats issued a subpoena on Thursday, shortly after the indictment was unsealed. Both men hired Trump's former attorney John Dowd to help them navigate the Capitol Hill inquiry.

Fruman was charged with four federal crimes, including violating campaign finance laws, lying to the Federal Election Commission and conspiracy.

David Correia

Correia, 44, is an American businessman and the only defendant in the case who was born in the United States. He was charged with one crime: conspiring to violate campaign finance laws by orchestrating political donations from a foreign donor.

According to the indictment, the defendants funneled donations from a Russian citizen to political candidates in Nevada and New York, among other states. This was part of a strategy to gain influence from these officials, who they hoped could help them secure licenses for a legal marijuana business. The planned company never got off the ground.

Prosecutors say It was Correia who maintained the records of the "multistate licensing strategy," tracking when the wire transfers would arrive from Russia and who would get the donations.

He has met Trump before, and posted photos of their meetings on social media.

Andrey Kukushkin

Kukushkin, 46, is an American businessman who was born in Ukraine. He allegedly coordinated with the other defendants to inject money from a wealthy Russian into US political campaigns.

Prosecutors said the defendants hid the fact that the political donations came from a Russian citizen, which violates federal law . According to the indictment, Kukushkin said they needed to hide the donor's identity because of "his Russian roots and current political paranoia about it."

Federal investigators obtained damning records where Kukushkin appears to acknowledge the straw-donor scheme in plain terms, saying that "money transferred by" the Russian national "was to support the very specific people & states ... in order to obtain green light for licensing."

Kukushkin was charged with only one crime in the indictment: conspiring to violate campaign finance laws by orchestrating political donations from a foreign donor. He was arrested in California.

The Russian business daily newspaper Vedomosti profiled Kukushkin in May as one of several Russian businessmen who are investing in the American cannabis market. According to the article, Kukushkin invested in legal marijuana dispensaries in California and Nevada starting in 2014.

Kukushkin set up his own company, Oasis Fund, the article said, which grows marijuana and sells products under its own brand. California business records say Kukushkin is a manager of the company, described as "an agriculture tech research and development" firm.