Prosecutors have sent target letters to the adult children of people charged in the college admissions scandal indicating they could also face criminal charges, a source familiar with the letters tells CBS News' Pat Milton. Target letters by prosecutors typically inform a person that they are part of an investigation but they don't necessarily mean the individual will be charged.

The people who were sent letters are believed to have known about the scam and who were at least 18 years old at the time, according to the person who spoke only on the condition of anonymity.

The letters were sent by the U.S. attorney's office in Boston. It's unknown how many letters were sent out.

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The U.S. attorney's office in Boston declined to comment.

More than a dozen people, including actress Felicity Huffman and the admitted mastermind of the scheme, have pleaded guilty in connection to the cheating scandal. Sixteen others, including actress Lori Loughlin and her fasion designer husband Mossimo Giannulli, are facing new charges. None of the students have been charged yet, but Yale has recinded admission of one student and Stanford has expelled students linked to the scandal.

After pleading guilty earlier this week, Huffman issued a statement saying her daughter knew "absolutely nothing" about her actions.

"My daughter knew absolutely nothing about my actions, and in my misguided and profoundly wrong way, I have betrayed her," the statement said. "This transgression toward her and the public I will carry for the rest of my life. My desire to help my daughter is no excuse to break the law or engage in dishonesty."

On Friday, Mark Riddell, who took standardized tests for some of the students, pleaded guilty to fraud and money laundering. Rick Singer, the admitted mastermind behind the scheme paid Riddell "$10,000 per test" to fly from his home in Florida to test centers in Texas and California. There, the Harvard graduate would "secretly take the exams in place of actual students" with some parents even providing samples of their children's "handwriting, so Riddell could imitate it while taking the exam."

In other cases, like with Huffman's daughter, he would simply alter test answers, to achieve a higher score. In her case, it was 400 points higher, but not too high, according to prosecutors, "in order to avoid any suspicion of cheating."