Federal prosecutors are probing whether President Trump's personal lawyer, Michael Cohen, acted as a secret lobbyist during previous business deals, The Wall Street Journal reported Thursday.

People familiar with the investigation told the outlet that prosecutors are reaching out to companies that Cohen worked for as a consultant after the 2016 election.

Some of these high profile companies include AT&T and Swiss drug company Novartis.

Cohen entered into contracts with companies during the transition period and bragged about his access to Trump's inner circle, according to previous reports.

The Wall Street Journal reported that investigators in the U.S. Attorney's Office for the Southern District of New York are probing these consulting deals to see if Cohen violated federal disclosure laws.

Federal law requires individuals to register as either a foreign or domestic lobbyist and fill out a disclosure form if they're communicating with public officials with the goal of push policy agendas on behalf of clients.

Failure to do so carries financial punishment and a potential prison sentence of up to five years.

The newspaper noted that it is common for consultants to be brought on to work with companies at the start of new administrations.

Consultants, however, cannot pitch elected officials about specific policies platforms without registering as a lobbyist.

Cohen used his consulting firm, Essential Consultants LLC, to enter into big-dollar contracts with AT&T and Swiss drug manufacturer Novartis. According to the companies, Cohen offered his access to and insight on the Trump administration in exchange for payments.

Essential Consultants is the same shell company Cohen used to pay $130,000 to adult-film star Stormy Daniels as part of a nondisclosure agreement meant to keep her quiet about an alleged affair she had with Trump.

It was revealed last month that Novartis paid Cohen $1.2 million for a one-year contract in which he promised to provide insight into Trump's health care policies.

The company's senior lawyer announced that he would leave the company over the payment, days after it was first reported.

AT&T reportedly paid Cohen's shell company four separate installments of $50,000, totaling $200,000, in late 2017 and early 2018, according to a document released by Stormy Daniels's lawyer Michael Avenatti.

The telecom giant did not disclose how much it paid Cohen but acknowledged the partnership.

Essential Consulting, however, "did no legal or lobbying work for us," the company said last month.

Bob Quinn, a senior executive at AT&T, announced that he was leaving the company shortly after reports of the payments were brought to light.

Special counsel Robert Mueller's office has reportedly been in contact with both companies.

The White House said last month that the payments prove that Trump will not cater to "special interests" from companies looking to change his mind.

"This is actually the definition of 'draining the swamp,'" White House press secretary Sarah Huckabee Sanders said.

Cohen is already under investigation for possible bank fraud and campaign finance violations. The FBI raided Cohen's home, office and hotel room in April as part of the probe.

ABC News first reported on Wednesday that Cohen will likely cooperate with federal prosecutors. The outlet also reported that the law firm representing Cohen will no longer work with him going forward.

The lawyer has denied any wrongdoing and has not been charged with any crime.

--Updated 6:34 p.m.