Court documents show FBI agents searched divorce records from Guam and studied the birth dates of Dotcom's children to fight off efforts by Mrs Dotcom to claim money seized almost three years ago.
FBI agent Rodney Hays testified the records showed Dotcom married his first wife, Lovely Vargas, three months before the date Mona Dotcom claimed her "de facto" marriage began.
Court papers said "inconsistencies" would make it harder for her to make a claim on her husband's fortune in New Zealand courts -- and cited a New Zealand government lawyer as an "expert witness".
Dotcom took to Twitter to voice his outrage, saying: "Frustrated US DOJ [Department of Justice] prosecutors attack my estranged wife Mona, her assets & the future of our children. You know you're winning when they start attacking the people you love."
The latest turn in the long-running cases follows Mrs Dotcom's attempt to register a claim on half of her husband's restrained fortune.
The US Department of Justice claims Dotcom's wealth stems from crime and should be surrendered.
The case runs alongside the FBI's prosecution of German-born Kim Dotcom on charges of criminal copyright conspiracy.
The new court records include documents from the Guam divorce court, showing Dotcom and Ms Vargas were married in August 2007 and had a child. The couple separated in September 2008 and were divorced in May 2009.
Two months later, the records show he married Mona Verga, as she was then, who had given birth earlier that year to his second child.
The importance of the date to the US prosecutors was a statement by Mrs Dotcom that she was entitled to half his property "as I began a de facto relationship with Kim Dotcom in November 2007".
The statement was contrasted with dates from divorce court records, along with New Zealand testimony and media coverage in which Mrs Dotcom said she "met Kim" in November 2007.
The Department of Justice told the US District Court in East Virginia Mona Dotcom could not use the date she met her husband to claim a de facto marriage when Dotcom was already married.
It also says Mrs Dotcom's claim on her husband's fortune should not extend to his collection of exotic and luxury cars because she can't drive.
It says the cars should have also given Mrs Dotcom a clue to her husband's allegedly illegal business model because of the personalised numberplates "Guilty", "Mafia", "Evil" and "Hacker".
"Her husband's lack of discretion should have put Mona Dotcom on notice to inquire as to the sudden source of her husband's wealth."
It also said Mrs Dotcom had already received more than the $23 million she had claimed, as Dotcom had said he had signed over the family trust to her control.
The court papers estimated the trust contained shares in the cloud storage Mega service worth $35 million.
The US authorities also sought and received support from New Zealand government lawyers to fight off Mrs Dotcom's claim.
Dr Mathew Downs, senior counsel at the Crown Law Office, told the US court Mrs Dotcom's claim would probably fail in New Zealand courts.
He said: "Courts have consistently refused to allow an offender's spouse to retain the benefits of criminal offending even if she is entirely innocent".
- By David Fisher of the New Zealand Herald
