Donald Trump has said US investigators are looking into how Jamal Khashoggi vanished at the Saudi consulate in Istanbul, but made clear that whatever the outcome, the US would not forgo lucrative arms deals with Riyadh.

The president’s announcement raised concerns of a cover-up of evidence implicating Saudi Arabia’s powerful crown prince, Mohammed bin Salman, in plans to silence the dissident journalist. Those fears were also heightened by an announcement that the Turkish and Saudi governments would conduct a joint investigation into the case.

The Saudi-owned al-Arabiya TV network described the 15 suspects as “tourists” who had traveled to Istanbul by commercial plane.

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Senior Republicans in Congress, briefed on US intelligence, have meanwhile signaled they were prepared to force the US to take punitive action if Khashoggi was found to have been murdered by the Saudi regime.

“We’re being very tough. And we have investigators over there and we’re working with Turkey, and frankly we’re working with Saudi Arabia. We want to find out what happened,” Trump told Fox News on Thursday morning.

US officials could not confirm that US investigators were in Turkey, which has hitherto resisted US help. The state department spokeswoman, Heather Nauert, said: “The US government has offered its support to the Turkish government to provide law enforcement assistance to the Turkish government.”

Nauert said she could not comment on whether there were US investigators “on the ground”. The state department had referred earlier questions about the case to the FBI.

Nauert revealed that the Saudi ambassador to Washington, the Crown Prince’s younger brother, Khalid bin Salman, had flown back to Riyadh on Thursday.

“We have said to him that we expect information upon his return to the United States,” she said.

Any sense that the administration might seek to impose serious consequences on Saudi Arabia was dispelled by the president. Asked at an impromptu press conference in the Oval Office whether the US would cut arms sales if the Saudi government was found to be responsible for Khashoggi’s disappearance, the president demurred, saying the US could lose its share of the huge Saudi arms market to Russia or China.

In the Oval Office Trump pointed out that the disappearance took place in Turkey and that Khashoggi was not a US citizen. On being told that the journalist was a US permanent resident, Trump said: “We don’t like it even a little bit. But whether or not we should stop $110bn [£83bn] from being spent in this country – knowing they have … two very good alternatives. That would not be acceptable to me.”

He continued: “I don’t like stopping massive amounts of money that’s being poured into our country – they are spending $110bn on military equipment and on things that create jobs for this country.”

The president’s desire to protect weapons sales and his family’s close relationship with the Saudi monarchy could lead to a clash with congressional Republicans, some of whom are already uneasy about the high civilian death toll from the Saudi aerial bombardment of Yemen, using US-made bombs.

The Republican chair of the Senate foreign relations committee, Bob Corker, one of a handful of senators briefed on US intelligence on the case, said he believed Khashoggi was murdered and that the “intel points directly” at the Saudi government. “I think they did it and unfortunately I think he is deceased. But they certainly could produce him and change the narrative,” Corker told CNN.

The senator told MSNBC he had seen intelligence in a secure room at the Senate and concluded: “It does appear that he’s been murdered, and I think over the next several days, things are going to become much clearer.”

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Corker and 21 other senators sent a formal letter to the president triggering a mandatory US investigation into Khashoggi’s fate. Under the Global Magnitsky Human Rights Accountability Act, the administration would have to report on the conclusions of the investigation and a decision on sanctions against the perpetrators.

On Thursday, a Turkish presidential aide, İbrahim Kalın, said there would be a joint Turkish-Saudi investigation into the Khashoggi case.

Turkish officials have released a relentless drip-feed of information about an alleged crime that has shattered diplomatic norms and rocked Ankara and Riyadh. A report in the Washington Post, citing US intelligence sources, said Bin Salman had earlier authorised an operation to lure Khashoggi back to Saudi Arabia and detain him.

On Thursday, the Post reported that Turkey had told the US that it has audio and video recordings that prove Khashoggi was killed inside the consulate.

“You can hear his voice and the voices of men speaking Arabic,” one person with knowledge of the recordings told the paper. “You can hear how he was interrogated, tortured and then murdered.” A second person said men could be heard beating the journalist.

The Turkish president, Recep Tayyip Erdoğan, has challenged Saudi Arabia to provide CCTV images to back up its claims that Khashoggi had left the consulate safely, indicating he did not find the current Saudi explanations sufficient.

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Turkish officials have told reporters that Khashoggi was killed soon after he entered the consulate last Tuesday by a hit squad of 15 assassins who had flown in from Riyadh that day. Accounts of his apparent death have been widely circulated by officials, who have released the names of the Saudi citizens who arrived on two private jets; all were connected to state security agencies.

The Middle East Eye website cited Turkish officials as saying that Khashoggi was ushered to the consul general’s office when he entered the consulate, then quickly seized by two men. “We know when Jamal was killed, in which room he was killed and where the body was taken to be dismembered,” the official said. “If the forensic team are allowed in, they know exactly where to go.”

Riyadh had previously pledged to allow Turkish officials into the consulate, which is considered sovereign Saudi territory under international convention. However, access was rescinded after the names of the alleged assassins were revealed. Among the group, according to a passenger manifest supplied by Turkish authorities, was the head of forensics for the Saudi General Intelligence Presidency.

While investigators believe Khashoggi was killed inside the consulate, suspicion about where his body may have been disposed of continues to focus on the Saudi consul general’s home, about 500 metres away. The building has an underground garage, and cars that were seen leaving the nearby building are believed to have spent several hours in the garage before leaving for Atatürk airport in Istanbul.

Officials also told Reuters they were examining data from an Apple Watch that Khashoggi was wearing when he entered the building. Central to the investigation is whether data from the smartwatch could have been transmitted to a cloud, or his personal phone, which was with his fiancee, Hatice Cengiz, who was waiting outside.

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Saudi officials had refused to engage with their Turkish counterparts until Tuesday, a source told the Guardian. Riyadh had used Washington as a conduit. “They have been behaving very strangely,” said an official. “It’s like they don’t care about the consequences. Is this incompetence, or arrogance? We really don’t know.”

On his first international trip as president, Trump visited Saudi Arabia and announced $110bn in proposed arms sales.

The US treasury secretary, Steven Mnuchin, is due to represent the administration at a trade and investment conference in Saudi Arabia next week, known as “Davos in the Desert”. His attendance would be a powerful gesture of support for Riyadh in the face of allegations of the premeditated murder of a US resident and journalist.

The US has no ambassador in either Turkey or Saudi Arabia.