It could soon be a very empty house for Lori Loughlin and her designer husband as a third family member is now the focus of a Department of Justice probe for her involvement in the college admission scandal that ensnared the actress.

Multiple sources tell DailyMail.com that one of the actress’ daughters received a target letter from federal prosecutors in Massachusetts earlier this month regarding the Operation Varsity Blues investigation.

That letter informed the embattled University of Southern California student that she was the subject of an investigation which could result in criminal charges.

No students have been charged or arrested at this point in the investigation, but court documents make it clear that some of the youngsters were aware of the illegal actions being carried out in their behalf.

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Fullest house: One of Lori Loughlin's daughters received a target letter from the Department of Justice announcing that they were the subject of a criminal probe (l to r: Olivia, Lori, Massimo and Isabella)

Morning Lori: The sources DailyMail.com spoke with only knew about one of the daughters getting a letter, the tone of which was described as 'ominous' (Loughlin above on April 3 heading into court in Boston)

'It is a not-so-veiled threat,' said one source who has seen the letter.

'[The US Attorney's Office for the District of Massachusetts] is making it pretty clear that they have evidence that very strongly suggests she knew of the illegal plot.'

The sources DailyMail.com spoke with only knew about one of the daughters getting a letter, the tone of which was described as 'ominous.'

That letter failed to bring Loughlin and her husband Mossimo Giannulli to the bargaining table however, with the pair entering a not guilty plea on Monday to charges of mail fraud and money laundering.

A lawyer for Loughlin did not respond to a request for comment, and it is not clear if her daughter has sought separate counsel.

A spokesperson for the US Attorney's office declined comment.

Shortly after their not guilty plea, their eldest daughter Isabella Giannulli deactivated her Instagram account.

One of the sources said that target letters have been going out for the past month as prosecutors work to obtain evidence in the case and pursue additional charges and defendants.

These latest target letters seem be less concerned with gathering information in the case said that source, who described the notes as 'warning shots' to the students.

'It is clear that some students are going to be charged,' said the first source.

That individual then added that they know five people who have received letters similar to Loughlin's daughter, noting that in all five cases the parent/s refused a plea deal and later filed a plea of not guilty.

Loughlin and her husband are facing jail time because they opted to use the athletics route to gain their daughters admittance into University of Southern California.

This required daughters Isabella and Olivia, who had never before rowed in their lives, to pretend they were on crew teams.

In order to sell that, they posed for photos on ergometers, suggesting that they were both aware and willing participants in their parents' plan.

Furthermore, Olivia knowingly had Singer's team fill out her college applications according to the complaint.

'On or about December 12, 2017, Loughlin e-mailed [Singer], copying Giannulli and their younger daughter [Olivia], to request guidance on how to complete the formal USC application, in the wake of her daughter’s provisional acceptance as a recruited athlete,' states the complaint.

'Loughlin wrote: "[Our younger daughter] has not submitted all her colleges [sic] apps and is confused on how to do so. I want to make sure she gets those in as I don’t want to call any attention to [her] with our little friend at [her high school]. Can you tell us how to proceed?"'

In response, Singer wrote an email 'directing an employee to submit the applications on behalf of the Giannullis’ younger daughter [Olivia].

Loughlin and Giannulli 'agreed to pay bribes totaling $500,000 in exchange for having their two daughters designated as recruits to the USC crew team - despite the fact that they did not participate in crew - thereby facilitating their admission to USC,' according to the documents.

The couple emailed Singer in 2016 about their daughters college prospects, stating that they wanted to do the necessary work to see that the girls got into USC as opposed to ASU.

Sister, sister: The 'DOJ is making it pretty clear that they have evidence that very strongly suggests she knew of the illegal plot' said another source (l to r: Olivia and Isabella)

Emails obtained by investigators revealed that it was soon decided that bribes would be paid to have the girls recruited as crew coxswains.

A short time after that, photos were taken of older daughter Isabella on an ergometer.

Dr. Donna Heinel, the senior associate athletic director at USC, presented Isabella as a potential athletic recruit a month later and she was approved for conditional admission.

It was then asked that a check be sent to Heinle for $50,000 from Giannulli, who a short time later asked if he could mention the story to the athletic director at USC when the two men were at Augusta, likely playing at the famed golf club.

Singer stated that this was not a good idea because when he discussed the possibility of Isabella's admission with him a year he thought the family 'would be good for a million plus.'

Isabella received her admission letter the following March, which was followed by another note requesting a donation of $200,000 to Key Worldwide Foundation.

After the amount was wired to the organization a note was returned that stated no good or services had been exchanged for the money, which according to the documents is false.

This same exact process was then repeated with the young daughter, Olivia, it is claimed.

Problems arose however when Olivia's guidance counselor became curious as to how she managed to receive admission based on her involvement in crew since she did not row.

At the same time, Loughlin complained that her daughter was having difficulty filling out her other college applications, prompting Singer to ask an employee to take care of that task.

This was done so as not to draw attention to the fact that it was already confirmed that she had received conditional admission to the school.

Then, at some point, there was a very heated and public altercation between Giannulli and the counselor, which elicited an email from Dr Heinel asking that this not happen in the future so as to avoid detection.

This all began to fall apart in October 2018 when the IRS audited Key Worldwide and began to look into donations made by parents whose children were then admitted to USC.

The two were told by Singer to say they had given the money to help 'underserved kids.'