Recent reports that special counsel Robert Mueller’s probe is focusing on the finances of Donald Trump and his associates prompted the President’s defenders to cry foul about what they see as a burgeoning “fishing expedition.”

But former federal prosecutors said looking at the financial relationships that connect Trump associates to Russia is par for the course for an investigation like this. They told TPM they would have been more surprised had Mueller not included those financial ties in his purview.

“It would seem to me impossible to investigate the collusion issue without looking to see what is the relationship between Trump, his business associates, and/or relatives and people and entities in Russia,” said Patrick Cotter, a former federal prosecutor who investigated organized crime for the U.S. Attorney’s Office of Eastern New York.

“You would have to. That is part and parcel of the investigation that Mueller is charged to look into,” he told TPM.

The federal probe now being led by Mueller is reviewing financial records related to the Trump Organization, as well as the financial dealings of various Trump associates and family members, CNN reported last week. Mueller, a former FBI director, has hired a number of former prosecutors who specialize in money laundering and other financial crimes to his team, and it also was reported last week that the probe is already working with grand juries in Washington, D.C. and Virginia.

A spokesperson for the Mueller probe declined to comment to TPM.

Trump had warned against Mueller delving into his family’s finances while examining Russian interference in the 2016 election in an interview with the New York Times last month. However, legal experts with experience in federal investigations told TPM that they expected the money trail to be critical to Mueller’s ability to understand Russia’s role in the presidential campaign.

“One way you show the collusion is you show motive and why somebody would do it. It shows motive and access,” said former U.S. Attorney Nick Akerman, who prosecuted the Watergate case.

All sorts of investigations, from drug trafficking to organized crime to espionage, examine the way money moves around, the former prosecutors said.

Rebecca Lonergan, a University of Southern California Gould School of Law professor who previously served in the U.S. Attorney’s Office in Los Angeles, explained that focusing on finances can help prosecutors in situations where other charges would be difficult to prove in court because they rely on classified information that would pose a threat to national security if made public.

“‘I’ve done this in espionage cases where there were sources and methods I couldn’t reveal,” Lonergan told TPM. “What I did is I charged financial crimes and showed that somebody who I knew was a spy for another country had $5 million in unexplained income and never paid taxes on it, and that income came from China. Do I reveal what they did for China and our sources and methods and how we know that? No. And I avoid that by the money. You got money, you didn’t pay taxes on it.”

“A lot of times the way you stay away from your classified information and being forced to reveal your sources and methods is you just track the money and the money speaks for itself,” she added.

More broadly, former prosecutors argued that the news that Mueller’s probe was examining financial relationships was standard operating procedure in any investigation like the one he took over.

“It is not only okay, it would be the headline if they weren’t looking at the financials. That would be weird. That would be bizarre. I don’t know how you would do it,” Cotter said.

Nonetheless, this aspect of Mueller’s investigation is being used by Trump and his allies to cast doubt on the entire operation, which has been so tight-lipped that it’s not confirmed who’s being investigated as part of the probe.

Jay Sekulow, a private lawyer for the President, told the Washington Post last month that Trump’s outside legal team would object if it thought the investigation was “drifting.” He cited a Bloomberg story about Mueller examining a real estate deal Trump made with a Russian oligarch in 2008.

“They’re talking about real estate transactions in Palm Beach several years ago,” Sekulow said. “In our view, this is far outside the scope of a legitimate investigation.”

Outside criticisms of Mueller’s investigation ramped up after last week’s CNN report. Asked about Mueller’s look into Trump’s finances on Thursday, White House counselor Kellyanne Conway said,“We know that these types of endeavors end up being fishing expeditions.”

Conservative commentator Byron York suggested that the focus on finances meant Mueller was having trouble finding evidence of any Trump campaign collusion with Russia:

This Russia collusion stuff is hard. Let’s try to get ’em on something else. From CNN; https://t.co/aUUg8IwHPH pic.twitter.com/SpZN2QHXdB — Byron York (@ByronYork) August 4, 2017

Deputy Attorney General Rod Rosenstein, who appointed Mueller as special counsel since Attorney General Jeff Sessions had recused himself from the Russia probe, pushed back Sunday against claims that the investigation had become a fishing expedition. In an interview on “Fox News Sunday,” he noted that if Mueller saw evidence of crimes outside the investigation’s original scope, he’d need Rosenstein’s permission before pursuing them.