A US-based lawsuit against Saudi Crown Prince Mohammed bin Salman centred on a Caribbean oil refinery, but unexpectedly highlighted something else -- the disappearance of his main rival.

Former crown prince Mohammed bin Nayef (MBN), unseated as heir to the throne by his cousin Prince Mohammed in 2017, has not been seen in public since his detention in March last year.

MBN, long seen as the CIA's most trusted Saudi ally, served as the kingdom's interior minister from 2012 before becoming crown prince three years later.

The lawsuit pointed to government efforts to keep a tight lid on his whereabouts, with documents showing the detained royal was represented by a US law firm that works for his rival.

The man behind the lawsuit, Saudi businessman Nader Turki Aldossari, is barred from leaving the kingdom along with his family members, according to letters from his lawyer to President Joe Biden and other US officials.

The story began in June last year, when Aldossari filed a lawsuit in Pennsylvania on behalf of his son Rakan, an American citizen, against MBN and other Saudi entities.

He alleged they had failed to honour a decades-old contract related to a refinery project on the Caribbean island of Saint Lucia.

But the case posed a peculiar conundrum: how to serve a summons to a prince whose whereabouts were unknown?

The suit was amended to include Prince Mohammed, stating that he had placed MBN under house arrest and seized his assets, thus preventing him from meeting his contractual obligations.

And when Aldossari said a summons could not be served against MBN, the court ordered Prince Mohammed's lawyers to help ascertain his location.

In March, Prince Mohammed's counsel offered to provide MBN's address on a "confidential basis", saying in a court filing that he faced terrorism-related threats due to his previous role as the kingdom's interior minister.

There was no mention of him being detained.

Aldossari's counsel insisted that Prince Mohammed was "holding the former crown prince under house arrest".

"Nayef is effectively a prisoner of... Saudi Arabia," he said in a filing.

But last month, the judge threw out Aldossari's breach-of-contract case, leaving the questions of MBN's status and whereabouts unresolved.

Crown Prince Mohammed bin Salman (L) unseated Mohammed bin Nayef (R) as heir to the Saudi throne in 2017Mohammed bin Nayef (R) speaking with deputy Crown Prince Mohammed bin Salman during the opening session of the Shura Council in Riyadh. A US-based
Crown Prince Mohammed bin Salman (L) unseated Mohammed bin Nayef (R) as heir to the Saudi throne in 2017Mohammed bin Nayef (R) speaking with deputy Crown Prince Mohammed bin Salman during the opening session of the Shura Council in Riyadh. A US-based lawsuit against Saudi Crown Prince Mohammed bin Salman centered on a Caribbean oil refinery, but unexpectedly highlighted something else, the disappearance of his main rival. Saudi Royal Palace / BANDAR AL-JALOUD

Aldossari's lawyer James Tallman told AFP he plans to appeal -- as well as fight his own clients' travel ban, which he fears "could escalate to detention".

Saudi authorities have not publicly commented on the detention of MBN, which laid bare rivalries inside the royal family.

Prince Mohammed has detained or sidelined multiple royal rivals in his rise to become the kingdom's de facto ruler.

But a particular target has been MBN, who enjoys "much more support within the royal family", said Bruce Riedel, a former long-serving American CIA officer.

MBN was abruptly removed from the royal line of succession in 2017 after King Salman appointed Prince Mohammed, his son, as crown prince.

Since MBN's detention, the government has been pursuing corruption and disloyalty charges against him, sources on both sides say.

In a report last December, a British parliamentary fact-finding panel said MBN had "not been able to contest his detention before an independent and impartial judge (and) has no access to a lawyer to discuss his situation".

But just days before the Aldossari lawsuit was dismissed, multiple lawyers from Squire Patton Boggs, a Washington lobbying and law firm, registered to represent MBN in the case -- while also working for his rival.

Squire Patton Boggs did not respond to AFP when asked if they had been directly recruited by MBN, whether they had access to him in detention or how they could simultaneously represent him and the government.

Squire Patton Boggs "have never represented MBN before," a source in direct contact with MBN's Europe-based legal team told AFP.

"MBN has never met them, and it is unclear how they would get access to him... when he has been denied access to his long-standing lawyers, due process, or any kind of communication with the outside world."

The Saudi royal court did not respond to AFP's request for comment on the law firm's involvement in the case and on MBN's detention.

Since 2016, Squire Patton Boggs has represented the Center for Studies and Media Affairs (CSMARC) at the Saudi royal court, receiving a total of around $2.7 million for the work, according to US justice department filings.

A recent US intelligence report said officials associated with CSMARC were involved in Saudi journalist Jamal Khashoggi's 2018 murder in Turkey.

Squire Patton Boggs also represents Prince Mohammed's co-defendants in a separate US lawsuit brought by former intelligence czar Saad Aljabri, a long-time advisor to MBN, according to US court filings.

A US-based source close to Aljabri said it was "incomprehensible to see lawyers appearing on behalf of MBN in one lawsuit while representing Prince Mohammed's co-defendants in another lawsuit."