Senate Armed Services Committee Chairman John McCain said Tuesday he does not think the trio of charges filed in the investigation led by special counsel Robert Mueller will be the last, and other lawmakers agreed.

"I guarantee you more shoes will drop," McCain, R-Ariz., told reporters.

Neither Republicans nor Democrats publicly addressed the new developments in the Mueller investigation during separate closed-door party lunches. But they have been discussing the matter with each other and with reporters.

Some noted the charges so far show no evidence President Trump worked with the Russians to influence the 2016 presidential election.

"Initially, all the facts coming out of this are unrelated to the campaign," said Sen. Jim Lankford, R-Okla., a member of the Senate Intelligence Committee probing Russian election interference.

But others say they anticipate more charges. McCain said he doesn't know who Mueller might be targeting next.

"I don't know," he said. "But they always have and they always will. I guarantee you this scandal is not over."

Mueller's probe has ensnared former Trump campaign chairman Paul Manafort; his business partner Rick Gates; and a campaign volunteer, who served with the title of foreign policy adviser, George Papadopoulos.

The charges against Manafort and Gates include money laundering and failure to register as foreign agents. Papadopoulos pleaded guilty to lying to federal agents about his interactions with foreigners tied to the Russian government.

Sen. Amy Klobuchar, D-Minn., said the Manafort charges didn't surprise many lawmakers given their knowledge that he had not registered as a foreign agent. But other lawmakers said they believe Mueller will attempt to use the charges against the trio to gain incriminating information about those higher up in the Trump universe.

"The people I talk to think this is the opening, that there is much more to come," Sen. Dianne Feinstein of California, the top Democrat on the Senate Judiciary Committee, said Tuesday. Her committee is also probing Russia's interference in the 2016 election.

McCain declined to draw a direct comparison to the Watergate scandal, which led to the resignation of President Richard Nixon. He said he does not believe Trump will try to remove Mueller.

"I think we've seen that movie before," McCain said, referring to Nixon's firing of the independent special prosecutor Archibald Cox that helped lead to the former president's ouster.

McCain added that he never heard of Papadopoulos, who Trump identified as a "low-level volunteer."

"I had no knowledge whatsoever of him," McCain said.