"AT&T hiring Michael Cohen as a political consultant was a big mistake," the company's CEO Randall Stephenson said Friday morning.

AT&T paid Cohen, President Trump's personal lawyer and fixer, $600,000 through a contract that ended in December 2017.

The payments are now under scrutiny in part because Cohen is under federal investigation.

"To be clear, everything we did was done according to the law and entirely legitimate. But the fact is, our past association with Cohen was a serious misjudgment," Stephenson wrote in a memo to employees.

Related: AT&T confirms it paid Michael Cohen for consulting on Time Warner deal

"In this instance, our Washington D.C. team's vetting process clearly failed, and I take responsibility for that," he added.

Stephenson announced that Bob Quinn, one of the executives involved in the Cohen deal, "will be retiring."

Quinn took charge of AT&T's legislative affairs operation in Washington less than two years ago. A source with knowledge of the matter said he is leaving under pressure.

Stephenson expressed regret to the company's employees, citing the negative attention around AT&T in recent days.

"To all of you who work tirelessly every day to serve customers and represent the brand proudly, thank you. My personal commitment to you is -- we will do better," he wrote.

Information about the business arrangement has trickled out for several days.

It was first revealed by Stormy Daniels' lawyer Michael Avenatti on Tuesday.

Avenatti said he was aware of four months of payments to Cohen totaling $200,000. But AT&T had Cohen under contract for virtually all of 2017 for $50,000 a month. On Thursday a source with knowledge of the matter confirmed that AT&T paid Cohen $600,000 for the year.

AT&T also provided new information about how the arrangement came about.

Cohen "approached" Quinn's Washington office of AT&T "during the post-election transition period and said he was going to leave the Trump Organization and do consulting for a select few companies that wanted his opinion on the new President and his administration -- the key players, their priorities, and how they think," AT&T said Friday.

These are the "insights" that AT&T previously said it hired Cohen for.

AT&T also said, "we didn't ask him to set up any meetings for us with anyone in the administration and he didn't offer to do so."

Another one of Cohen's corporate clients, Novartis, offered a similar explanation.

Novartis also said Cohen approached them and offered his consulting services.

"We made a mistake in entering into this engagement and, as a consequence, are being criticized by a world that expects more from us," Novartis CEO Vasant Narasimhan said in a memo to employees.

AT&T also confirmed Friday afternoon that a separate consulting firm run by former Trump campaign staffers approached AT&T and offered its services.

That firm was Avenue Strategies, a political consulting firm co-founded by former Trump campaign manager Corey Lewandowski and campaign adviser Barry Bennett. (Lewandowski has since left the company.)

"Avenue Strategies called us to set up a meeting to pitch us on their services in early 2017," an AT&T spokesperson said in a statement. "We agreed to meet with them. We met with them once, and decided not to use them."

Avenue Strategies also denied ever working with AT&T, saying in a statement "Avenue Strategies has never worked for AT&T. We have never prepared any proposals for AT&T. Any claims to the contrary are simply not true."

Bennett said in a brief interview that he never spoke or dealt with AT&T, though he said a former Avenue Strategies employee may have spoken with them.