Former Secretary of State John Kerry has admitted meeting with Iranians who sponsor worldwide terror without permission from the Trump administration.

And now at least one U.S. senator is calling for an investigation to determine whether or not Kerry committed crimes under the Logan Act or the Foreign Agents Registration Act.

Fox News reports it obtained a copy of a letter from Sen. Marco Rubio, R-Fla., to Attorney General Jeff Sessions stating Americans "deserve to know that U.S. laws are enforced regardless of any individual's past position."

He is asking for an investigation into Kerry's meeting with the Iranian foreign minister during the Trump presidency, which the president already has suggested could be a crime.

Under the Logan Act, unauthorized personnel are banned from negotiating with a foreign government in relation to any disputes with the U.S. And under the registration act, people who are acting on behalf of foreign governments, political parties or others much disclose their activities.

After reports of Kerry's meetings with the Iranian official, Mohammad Javad Zarik, the former secretary of state acknowledged they had occurred.

Kerry said he had met several times with Iranian officials in Norway and Germany, and at the United Nations headquarters.

"While meetings between private U.S. citizens and foreign officials are not illegal or in violation of federal regulations, Josh Rosenstein, a partner with the Washington law firm Sandler Reiff and specialist in lobbying compliance, told The Associated Press there are too many unanswered questions – specifically in regards to where the meetings took place and what was specifically discussed," Fox News said.

"The devil's always in the details," Rosenstein said. "Simply offering advice to a foreign government doesn't make you a foreign agent."

Kerry claimed he was trying to find out from the Iranian what his nation would be willing to do to "change the dynamic in the Middle East."

President Trump wasted no time when the meetings were revealed, tweeting: "John Kerry had illegal meetings with the very hostile Iranian Regime, which can only serve to undercut our great work to the detriment of the American people. He told them to wait out the Trump Administration! Was he registered under the Foreign Agents Registration Act? BAD!"

Trump withdrew the United States from Kerry's negotiated nuclear weapons deal with Iran after concluding it wouldn't prevent Iran from obtaining nuclear weapons.

Secretary of State Mike Pompeo has accused Kerry of "undermining" U.S. policy by holding meetings with Zarif since leaving office.

"You can't find precedent for this in U.S. history, and Secretary Kerry ought not to engage in that kind of behavior," Pompeo said.

The Washington Free Beacon reports that since Trump announced the U.S. was leaving the Iranian nuke deal, the Iranian economy "has continued to sputter and businesses and banks have exited its marketplace."

"Rising prices and domestic discontent have led to wide-scale protests against the Iranian regime."