The tricky nature of investigating Gen. Allen's emails

By Barbara Starr

A U.S. official familiar with the content of the e-mails Gen. John Allen sent to Jill Kelley described them as "embarrassing" and warranting the investigation that has been launched.

"They are embarrassing. If they got out, John Allen would be very embarrassed by them," according to the official, who emphasized that there is no evidence of "physical contact between the two."

The official noted that the e-mails that are under investigation - which have been called "potentially inappropriate" - are from Allen. It is Allen's e-mails that are under investigation, and the official did not know the content of any of Kelley's e-mails to Allen.

"The e-mails are concerning enough to warrant an investigation," the official said.

Related: General John Allen: A Marine of distinction

A second U.S. official, who has had the e-mails described to him, characterized the content as "sexy," but emphasized he could not say whether they "crossed the line."

They "are not the type of e-mails you would want your wife to read," the second official explained.

But a senior official close to Allen told CNN's Nick Paton Walsh Tuesday that there was nothing pointing or referring to sex or an affair or anything of a romantic nature. Allen may have said, informally, "thanks sweetheart" in an e-mail, the official close to Allen insisted.

"Anyone who knows him knows his style - he has a habit of replying to every single e-mail (he is sent). Kelley would e-mail his business and personal accounts," that official said.

It will be up to the Defense Department’s inspector general to decide specifically if the content of the e-mails is so inappropriate that they would be considered conduct unbecoming an officer, according to a third source, a senior U.S. official. This official pointed out this may be a very difficult matter to decide because everyone has a personal definition of what is "affectionate, admiration, all the way to what some might define as sexting."