(CNN) Special counsel Robert Mueller has copies of vitriolic and sometimes threatening messages that Roger Stone directed at Randy Credico, a witness in the investigation, according to sources familiar with the investigation.

Investigators have been examining the text messages and emails and questioning witnesses about whether there was an attempt to harass or intimidate Credico, according to people familiar with the matter. Stone, a longtime political adviser to President Donald Trump, has claimed that Credico, a progressive New York political activist and radio host, served as his backchannel to WikiLeaks during the 2016 campaign. Credico has denied that he acted as an intermediary.

CNN has obtained copies of some of the messages. In one exchange, Stone responds to the news that Credico received a subpoena from Mueller

"Waste of your time -- tell him to go f--- himself," Stone wrote.

"Who?" Credico asked.

"Mueller," Stone said.

Other messages in April show Stone accusing Credico of cooperating with Mueller.

"Everyone says u are wearing a wire for Mueller," Stone wrote in one message. Another said: "vital people know you are wearing a wire and narching for Mueller to f--- Assange."

The messages raise the possibility that Mueller could pursue obstruction or witness tampering charges against Stone. Stone said he has not been contacted by Mueller's investigators. But nearly a dozen of his current and former associates have been summoned for interviews or for testimony before Mueller's grand jury

"Our text and email messages have to been seen in their totality to see the (sic) is no serious effort to intimidate or pressure Randy to do anything, other than fess up to the truth," Stone said in a statement to CNN, adding that he encouraged Credico to tell the truth in other messages.

Stone said his comments about wearing a wire were a joke and added he didn't believe Credico had actually been subpoenaed.

Mueller's team charged Paul Manafort and an associate with conspiracy and obstruction of justice for reaching out to potential witnesses after Manafort was indicted on tax and other charges and attempting to coach their possible testimony.

Manafort pleaded guilty after he was convicted on eight criminal charges and is cooperating with the special counsel's investigation

"For me, you have to distinguish between the sort of spurned lover and the person that's truly intending to act or make a true threat in respect to what that person may have to offer the special counsel," said CNN legal analyst Michael Zeldin.

But even if individual messages don't amount to obstruction, witness tampering or threats, if altogether "they paint a picture of a guy who's trying to influence a witness who may have testimony against him, then that's more problematic," Zeldin said.

A spokesman for the counsel's office declined to comment.

"Communications between Randy and Roger Stone to the extent that we have them have been provided to the special counsel," said Martin Stolar, an attorney for Credico. "I'm not going to confirm the contents of any of them."

Stone and Credico have a complicated history that spans more than a decade. Over the years, they've worked together on political campaigns and called each other friends. But it has been a rocky friendship. For instance, at one point years ago, Stone started a rumor that Credico was dead.

Stone said he identified Credico as his backchannel reluctantly, which sparked outrage from Credico.

"When I told Randy that I might be forced to disclose his name as my lawyers were recommending after my appearance before the House Intelligence Committee, he did not react well. In fact he was furious saying I should be willing to go to jail rather than reveal his name," Stone previously told CNN. "At no time did I ever suggest he not co-operate with any formal investigation or plead the 5th or do anything other than tell the truth."

The trove of messages between Stone and Credico demonstrates their fraught relationship. Both men hurl insults at one another via text message and email. Stone regularly slams Credico -- who has a history of drug and alcohol abuse -- as an alcoholic.

"Choke in your own vomit," Stone said in exchange. Credico hit back, accusing Stone of being a drug addict and a liar.

Asked about the drug accusations, Stone told CNN his efforts to legalize marijuana are hardly a secret, but "I actually prefer a cocktail."

"Our texts are the late night bickering of two grumpy political veterans, who have known each other for years. They are friendly as well as nasty, ribald, vicious, inconsistent humorous and vile," Stone told CNN.

Scattered among the venomous messages are also glimpses of their longstanding friendship, including dinner invitations and an offer from Stone to help Credico find work.

The saga that has ensued around the Mueller probe has contributed to the deterioration of their friendship.

In one caustic exchange between the two men, Stone calls Credico a "rat" and threatens to take away his dog, a fluffy white Coton de Tulear named Bianca. Bianca accompanied Credico to his grand jury appearance in September, with the permission of the special counsel's office.

"You backstab your friends-run your mouth my lawyers are dying Rip you to shreds," Stone wrote. "I'm going to take that dog away from you. Not a f------ thing you can do about it either because you are a weak broke piece of s---."

Credico replied, "You don't have a constitutional right to threaten me and especially not threaten my dog ...you crossed a red line."

"Rot in hell," Stone wrote back.