A lawyer for Hatice Cengiz, the fiancee of Jamal Khashoggi, has accused Mohammed bin Salman of engaging in an unprecedented and blatant attempt to "manipulate" the US court system in order to "secure impunity" after allegedly ordering the 2018 murder of the journalist.
In a blistering 10-page legal filing, lawyer Keith Harper, who represents Cengiz and the pro-democracy group Dawn in a US civil case against the Saudi crown prince, urged Judge John Bates to reject a controversial suggestion by the Biden administration that Prince Mohammed be granted sovereign immunity in the case.
Harper said that while it was customary for judges to defer to the executive branch on judgments of whether foreign leaders should be granted head-of-state immunity, this case "differs fundamentally" because - he said - Saudis had engaged in a legal manoeuvre that had no precedent "in the history of international law".
"In this rarest of cases, the court should decline to shield MBS for his ordering of the murder of US-resident Jamal Khashoggi," the filing said.
Cengiz and Dawn sued Prince Mohammed and his associates in 2020, accusing him of conspiring with premeditation to kidnap, torture and murder Khashoggi in the Saudi consulate in Istanbul in 2018.
Prince Mohammed's lawyer, Michael Kellogg - who has represented Saudi Arabia since the 9/11 attacks on the US - has argued that Cengiz did not have standing to bring the case against his client, whether or not one believed he was guilty of ordering the murder.
In June, Judge Bates asked the Biden administration to weigh in on the matter, and invited the US government to give its own opinion about whether the crown prince deserved to be treated as a head of government or state, which in most cases would lead the case to be dismissed.
After asking for two extensions, the Biden administration late last month complied with the request, and said in a legal filing that it did believe Prince Mohammed ought to receive immunity from the court, citing a decision by Saudi's King Salman - Prince Mohammed's father - to appoint his son prime minister.
The appointment of Prince Mohammed as prime minister - a post traditionally held by the king in Saudi Arabia - was announced just days before an earlier court-appointed deadline for the US to offer its opinion. Some human rights defenders saw the move as an attempt by the Saudi government to try to convince the court to confer immunity on the Saudi heir.
On Tuesday, Prince Mohammed's lawyer argued that the case was all but closed, and that the Biden administration had, in effect, divested the court of its jurisdiction on the issue.
While experts have said it is unlikely that Judge Bates would dismiss the Biden administration's views, Cengiz's lawyer urged the judge to do so, suggesting that doing anything else would make a mockery of the court itself.
While the administration had a right to offer its opinion, Harper argued, "that decision does not compel this court to accede to MBS's blatant attempt to manipulate this court's jurisdiction and thereby secure impunity for the horrific murder he ordered."
It was apparent, Harper added - and neither Saudi Arabia nor the US have "seriously disputed" - that the appointment was made "with the deliberate attempt to manipulate this court's jurisdiction. No comparable appointment has occurred in the history of international law."
To bolster his case, Harper argued that in no other case in history had a defendant been appointed to a high office for the sole purpose of being granted sovereign immunity.
Moreover, he said, the royal order that established Prince Mohammed as prime minister affirmed that the king - not the crown prince - would continue to chair sessions of the council of ministers.
He also argued that granting the Saudi prince immunity would not "promote international comity", which has traditionally been used to justify the legal principle.
"There is no way the United States, or any nation governed by the rule of law, would ever seek immunity under the same or similar circumstances," Harper argued. "In the United States, the president cannot autocratically declare that a family member is the 'head of government' in order to evade accountability in a foreign court."
The parties are due to meet for a hearing on the matter on 9 December in Washington.