The CIA’s conclusion that the murder of Jamal Khashoggi was ordered by Crown Prince Mohammed bin Salman has been described as the most damaging blow yet to the de facto Saudi leader, officially placing him at the heart of a scandal that continues to shake the region.

The finding, first reported by the Washington Post, for which Khashoggi had written, was the first US government assessment linking Prince Mohammed to the dissident Saudi’s death in Turkey.

It came after Donald Trump and his national security adviser, John Bolton, had tried to shield him from criminal investigations that have incriminated 21 Saudi agents in the grisly murder.

Profile Who was Jamal Khashoggi? Show Jamal Khashoggi was one of the Arab world’s most prominent journalists and commentators. He was an outspoken critic of Saudi Arabia who dared to defy Crown Prince Mohammed bin Salman. While living in Saudi Arabia, Khashoggi was told to stop writing or posting on Twitter, where he had more than 1.6 million followers. He moved to the US in June 2017, where he continued to comment on his country both in print and on television. He wrote columns for the Washington Post and the Guardian.

His message struck a nuanced tone in the US, where he tried to acknowledge the reforms undertaken by Bin Salman while also highlighting the flaws. Khashoggi previously had close links with the Saudi royal family, including having served as a media aide to Prince Turki al-Faisal, when the latter was director general of the Saudi intelligence agency. He was also a former editor of the Saudi newspaper al-Watan and had worked with Prince Alwaleed bin Talal, a grandson of the first Saudi king. Khashoggi was murdered in the Saudi Arabian consulate in Istanbul in October 2018. Photograph: Virginia Mayo/AP

The US president had previously suggested the killing inside the Saudi consulate in Istanbul was carried out by rogue actors who had exceeded their authority.

The CIA found, however, that Prince Mohammed’s tight grip on decision-making made such a claim highly unlikely.

A European intelligence official told the Observer the CIA’s finding was “highly damaging to the kingdom’s official narrative”.

Donald Trump discussed the CIA assessment by phone with the agency’s director, Gina Haspel, and secretary of state Mike Pompeo on Saturday, White House spokeswoman Sarah Huckabee Sanders told reporters. Trump, visiting California, called the CIA assessment “very premature” but said it was “possible” Prince Mohammed was responsible. The State Department spokeswoman Heather Nauert said: “Recent reports indicating that the US government has made a final conclusion are inaccurate.” As has become customary in the US administration’s response, she and Trump underlined the importance of the US-Saudi strategic relationship.

Formerly a strong supporter of the prince, Trump has appeared frustrated at the Khashoggi case and the complicity of senior officials, which has mapped a path to the doors of the royal court. Both Republican and Democratic senators on Saturday urged Trump to be tough on the crown prince. “Everything points to the Crown Prince of Saudi Arabia, MbS, ordering Washington Post journalist Jamal Khashoggi’s killing. The Trump administration should make a credible determination of responsibility before MbS executes the men who apparently carried out his orders,” tweeted Senator Bob Corker, chairman of the Senate foreign relations committee.

The Democratic senator Richard Blumenthal tweeted: “Trump must accept for once his intelligence experts’ incontrovertible conclusion: Crown Prince MBS is culpable for Khashoggi’s monstrous murder. This brazen killing must have consequences – sanctions, prosecution, removal of MbS and others, not continued cover-up, enabled by Trump.”

The CIA claim came after six weeks of relentless pressure on the kingdom and the crown prince, who has denied any knowledge of the hit squad sent to Turkey or its mission.

A European intelligence official said the finding from a top intelligence agency was “highly damaging to the kingdom’s official narrative”.

The Washington Post reported that the agency strengthened its conclusions after reviewing two intercepted calls. One was reportedly between Prince Mohammed and his brother Khalid bin Salman, the Saudi ambassador to Washington, and captured the former discussing ways to get Khashoggi back to Riyadh.

The second call reportedly captured Prince Khalid assuring Khashoggi that it was safe to visit the consulate in Istanbul.

Khashoggi arrived there on 2 October and never left. A Turkish-led investigation has found that he was killed and dismembered within seven minutes of entering the building. His body has not been found.

Prince Khalid strongly denied having been in contact with Khashoggi in the past year, and called on US officials to table their evidence.

“The last contact I had with Mr Khashoggi was via text on Oct 26 2017,” he said on Twitter. “I never talked to him by phone and certainly never suggested he go to Turkey for any reason. I ask the US government to release any information regarding this claim.”

The Saudi position has moved throughout the investigation. An initial blanket denial of any involvement, coupled with an assurance from Prince Khalid that Khashoggi walked freely from the consulate, gave way around 10 days later to a grudging admission that he had been killed in a fist fight, then to a concession that the killing had in fact been premeditated.

On Thursday, the Saudi state prosecutor claimed Khashoggi had died from an overdose of a drug injected into him by agents.

Turkey has attempted to maintain pressure on its regional rival, and Prince Mohammed in particular, by continuing to leak details of the investigation.

During the week, Turkish officials told government-aligned media that audio tapes existed of the conspirators going over their roles as they waited for Khashoggi.

The existence of tapes remains central to the investigation. Turkish intelligence officers are believed to have had a long-term bug inside the Saudi consul general’s office, where the first stages of the ambush occurred. A streaming call is also believed to have taken place at some stage during the killing.

After Khashoggi had died, a member of the kill team reportedly called an aide to Prince Mohammed and said: “Tell your boss the deed was done.” The contents of this call have added weight to the CIA’s assessment.

The western intelligence official said it was unusual for an intelligence agency to be so definitive in such a sensitive matter.

“The fact that they have means that they really do hold solid material. These intercepts will cause him [Mohammed] problems.”