(CNN) The US has intercepts of Saudi officials discussing a plan to lure journalist Jamal Khashoggi back to Saudi Arabia and detain him, according to a US official familiar with the intelligence.

Khashoggi, a prominent Saudi journalist and critic of the regime, has been missing for more than a week after going to the Saudi consulate in Istanbul for an appointment that he had scheduled to obtain wedding papers. Turkish officials privately believe he was killed at the consulate, an allegation denied by Saudi Arabia, which says he left the consulate the day he arrived.

The official said it is unclear if the original plan was to murder Khashoggi or if something went wrong at the consulate and that he might have been killed during an attempt to kidnap him. Getting Khashoggi to the consulate appears to have been a backup plan, the source said, because he couldn't be persuaded to fly back to Saudi Arabia.

The official said there is no hard evidence as to whether Khashoggi, a columnist for the Washington Post, is dead or alive.

The source did not say when the United States became aware of the discussions. As CNN reported earlier this week, intercepted communications were being reviewed in the wake of Khashoggi's disappearance.

The official would not go so far as to say Saudi Crown Prince Mohammed bin Salman ordered the operation, but said that, although he may not have known the specifics, such a plan couldn't have taken place without his approval.

The Washington Post first reported the details of the intercepts.

Khashoggi was in regular contact with Saudi authorities in 2017 about taking on a more prominent role in the Kingdom, including discussions about government support for a think tank he would lead, a source who knows the journalist told CNN.

Khashoggi rejected the ideas proposed to him by the main interlocutor, a senior adviser within the Royal Court, according to the source. But that dialogue came to an end over the following months with Khashoggi's increasingly sharp criticism of the government, particularly its domestic policy and the crisis with Qatar.

Senior Royal Court figures in Riyadh were especially infuriated by Khashoggi's criticism of the Saudi government's September 2017 decision to classify as terrorists the Muslim Brotherhood and the Islamic scholar Yusuf al-Qaradawi, according to the source, who maintains high-level contact inside the kingdom.

Khashoggi at the time became more wary of returning to the kingdom, the source said.

US officials think it's possible the Crown Prince wanted Khashoggi silenced, but miscalculated the global impact his disappearance would have.

Friends of Khashoggi, who have asked to stay anonymous for fear of being connected to him, have told CNN that there was an active campaign by the Saudi government and pro-Saudi government think tanks in Washington to convince him to stop criticizing bin Salman and get him back in the fold as an ally of the royal court.

One US-based friend told CNN that when Khashoggi would go to the Saudi embassy in Washington, officials would be nice to him and try to convince him that it would be safe to return home.

There is, however, no indication that this wooing campaign was connected to any plot to kidnap or kill Khashoggi.

An international affair

Earlier this week, bin Salman contacted the White House after it became clear he and the royal court were getting blamed for Khashoggi's murder, according to a person familiar with the call. Bin Salman asked specifically to speak with Jared Kushner, US President Donald Trump's son-in-law and senior adviser, to deny the accusations.

White House national security adviser John Bolton also joined the call, and Secretary of State Mike Pompeo had a separate conversation with the Crown Prince, sending a signal to bin Salman that he could not circumvent Trump's Cabinet officials.

US President Donald Trump is facing increased pressure over the Khashoggi case as he walks the US-Saudi relationship tightrope. Late Tuesday, a bipartisan group of senators wrote to the President, calling for the White House to determine what happened to Khashoggi and whether sanctions should be imposed on whoever was responsible for his fate.

The letter, penned by the top Republican and Democrat on the Senate Foreign Relations Committee, triggers an investigation under legislation that allows the President to impose sanctions on individuals or countries that are deemed to have committed a human rights violation. The White House must respond within 120 days, setting out what actions it proposes to take.

Trump told "Fox and Friends" during an interview on Thursday morning that his administration was being "very tough" with Saudi Arabia as it investigates Khashoggi's disappearance.

"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 said, adding that US inquiries were making progress. "We're probably getting closer than you might think," he said.

Asked whether the United States has investigators on the ground in Turkey, the FBI would not confirm or comment. A Turkish diplomatic official has denied that the United States has sent investigators to Turkey to look into the Khashoggi case, saying the reports are "not true."

On Wednesday night, Trump told Fox News that it was too early to say how the United States might respond, but that blocking further arms sales to Saudi Arabia "would be hurting us."

A German diplomat who knows the missing journalist said he was stunned by the apparent lack of urgency from the United States one day after Khashoggi's disappearance, according to a senior diplomatic source. The German Deputy Chief of Mission in Washington immediately called several people in the US administration, including the State Department, and was surprised by their response.

"It seemed they weren't aware, and didn't see it as such a big deal," according to the source familiar with the phone calls.

On Thursday, State Department spokesperson Heather Nauert said the Saudi ambassador to the United States, Khaled bin Salman, was on his way back to Saudi Arabia, but not at the request of the Trump administration. She said the State Department had been in touch with the ambassador and expected information from him upon his return.

"As we have said from the beginning, we are not sure what happened to Jamal Khashoggi," she said. "We are all concerned about his whereabouts. We do not have information on his whereabouts or what happened to him."

Investigation continues

On Thursday, a Turkish presidential spokesperson said that a joint working group would be established "at the request of Saudi Arabia," according to Turkey's state-run Anadolu news agency.

Earlier in the day, Turkish President Recep Tayyip Erdogan told reporters that Turkey "cannot remain silent" on the disappearance of Khashoggi.

"We are currently investigating the event in all its dimensions. The event took place in our country. We cannot remain silent in the face of an event like this because it is not a normal occurrence," Erdogan said while returning on a flight from Hungary. He said security and intelligence forces "have been looking at all dimensions of this" and are looking especially closely at the country's entry and exit points.

Timeline of Khashoggi's disappearance Friday, September 31 Khashoggi goes to consulate to obtain papers to allow him to get married The papers weren't ready and he leaves consulate after a couple of hours Tuesday, October 2 3:00 a.m.:Private jet arrives in Istanbul from Riyadh 1:14 p.m.: Khashoggi arrives at the consulate 5:00 p.m.: A second jet arrives in Istanbul from Riyadh 6:00 p.m.: Second flight departs Istanbul for Cairo, en route to Riyadh 11:00 p.m.: First flight departs Istanbul for Riyadh Wednesday, October 3 Turkish officials first report Khashoggi is missing

Turkish officials have said that a 15-man team flew from Saudi Arabia into Istanbul on the day Khashoggi entered the consulate, and were present in the building at the same time as the journalist.

A Saudi source familiar with four of the 15 men told CNN that one is a former diplomat in London and an intelligence officer, and another is a forensics expert.

In a statement to CNN on Wednesday, a Saudi official said the kingdom "categorically" denies "any involvement in Jamal's disappearance."

"At this stage, our priority is to support the investigation, as opposed to responding to evolving comments not directly related to those efforts. Jamal's well being, as a Saudi citizen, is our utmost concern and we are focusing on the investigation as a means to reveal the truth behind his disappearance. Our sympathies go out to the family during this difficult time," the official said.

But a senior Turkish official speaking on the condition of anonymity told CNN that the Saudis were not cooperating with the investigation: "They are not open to cooperating."

Hatice Cengiz, Khashoggi's fiancée, told CNN that although she is afraid that media reports of his death could be true, she wants to wait for a "final result" and still thinks "anything" could have happened to him.

Cengiz told the Turkish news agency Anadolu that Khashoggi's first visit to the consulate on the Friday before he went missing had gone smoothly.