Federal prosecutors are expected to file additional charges against some defendants who pleaded not guilty in connection to the college admissions scandal, a law enforcement official tells CNN. 

The additional charges, which are said to include bribery, could be filed as early as Tuesday, according to the official. 

An attorney for defendant Jovan Vavic, a former University of Southern California water polo coach, said he pleaded not guilty to conspiracy to commit racketeering because "he is innocent." 

"Nothing has changed," attorney Stephen Larson said. 

Gordon Ernst, a former tennis coach at Georgetown University who is facing the same charge, also pleaded not guilty. 

"My clients continue to rely on their constitutional right to a trial and will not be bullied into a plea on the threat of additional charges for exactly the same conduct," attorney Tracy Miner said. 

An attorney for Amy Colburn and Gregory Colburn, two parents both charged with conspiracy to commit mail fraud and honest services mail fraud as well as money laundering conspiracy, said they won't be changing their not guilty plea either. 

"Adding new charges at this time seems to be vindictive and intended to discourage our clients and others from exercising their constitutional right to a fair trial," attorney Patric Hooper said. 

The possibility of future charges may explain why three parents who initially pleaded not guilty agreed late last week to plead guilty

Douglas Hodge, Manuel Henriquez and Michelle Janavs have each agreed to plead guilty to conspiracy fraud and conspiracy to commit money laundering, according to the US Attorney's Office in the District of Massachusetts. All three have plea hearings for Monday. 

The threat of future charges reflects prosecutors' carrot-and-stick approach to this case as they use potential charges to try to get defendants to plead guilty. 

"The carrot is, 'Take a quick plea and get your best shot at a lower sentence," CNN legal analyst Elie Honig told CNN in April. "And the stick is, 'We have additional charges that we'll bring if you don't plead by that date." 

Prosecutors initially charged more than 30 parents with conspiracy fraud in March. Those who fought that charge, including actress Lori Loughlin and her husband Mossimo Giannulli, were then charged with a count of conspiracy to commit money laundering in a superseding indictment in April.

At least 50 people involved in the case

Federal prosecutors say at least 50 people were involved in a nationwide fraud to get students into prestigious universities, including wealthy parents, Hollywood actresses, coaches and college prep executives. Ten parents have been sentenced, including actress Felicity Huffman.

Hodge, the former CEO of the Pacific Investment Management Company (PIMCO), is accused of agreeing to pay $200,000 to facilitate his daughter's admission to the University of Southern California as a soccer recruit and submitted false soccer credentials on her application. He allegedly paid Rick Singer — the alleged mastermind of the scheme — another $325,000 to help his son get admitted to USC as a purported football recruit, prosecutors said.

Henriquez, the former CEO and founder of Hercules Capital, and his wife Elizabeth are accused of participating in the test-cheating aspect of the scheme four separate times for their two daughters. They also are accused of conspiring to bribe Georgetown's tennis coach to get their daughter into the university as a tennis recruit. Elizabeth Henriquez is still listed as pleading not guilty, according to the website of the US Attorney for the District of Massachusetts.

Janavs, a former food executive whose family's company invented Hot Pockets, is accused of paying to cheat on her daughter's ACT and of conspiring to bribe USC to get her daughter admitted as a beach volleyball recruit.