The senators, including Foreign Relations Committee Chairman Bob Corker and top Democrat Bob Menendez, sent the letter triggering a probe into the disappearance of Washington Post writer Jamal Khashoggi under the Global Magnitsky Human Rights Accountability Act.
The letter demands that the White House "determine whether a foreign person is responsible for the extrajudicial killing, torture or gross violation of internationally recognized human rights."
The Trump administration must report to Congress within 120 days. It could pave the way for sanctions, including against high-ranking officials in the Saudi government.
The move from Congress ratchets up pressure on Saudi Arabia to explain what happened to Khashoggi, a US resident and critic of the kingdom who disappeared after entering the country's consulate in Istanbul on October 2.
Turkish officials have said they believe Khashoggi was killed inside the consulate and his body removed. Saudi Arabia denies knowing anything about his fate, saying that he left the building. There is no footage of Khashoggi leaving the consulate, where his Turkish fiancee was waiting for him.
Trump told reporters in the Oval Office on Wednesday that he had talked "more than once" and "at the highest levels" with Saudi officials.
"It's a very serious situation for us and this White House," Trump said. "We do not like seeing what's going on. Now, as you know, they're saying, 'We had nothing to do with it.' But so far everyone's saying they had nothing to do with it."
Trump added: "Nobody knows what happened yet. We don't know what happened yet."
'Everything points to them'Saudi Arabia is one of the United States' strongest allies in the region. The Trump administration's Middle East policy, ranging from issues relating to Iran and Syria to Israel, has been built around the Saudi alliance despite criticism of the kingdom's abysmal human rights record and the war in Yemen.
Read more: UN details possible war crimes in Yemen conflict
Earlier Wednesday, Corker, who as the chairman of the US Senate Foreign Relations Committee has seen US intelligence reports, said that "the likelihood is he was killed on the day he walked into the consulate" and that "there was Saudi involvement" in whatever happened to Khashoggi.
"The Saudis have a lot of explaining to do because all indications are that they have been involved at minimum with his disappearance," Corker told The Associated Press. "Everything points to them."
TheWashington Post reported that US intelligence had intercepted communications of Saudi plans to kidnap Khashoggi, a critic of the kingdom and Crown Prince Mohammed bin Salman who fled into exile last year.
Turan Kislakci, a friend of Khashoggi who heads the Turkish-Arab Media Association, told DW that the Saudi critic "was killed because of his opinion about the wars in Yemen and Syria."
"The Saudi government isn't comfortable with him. So they tried to get rid of him," Kislakci said. "He was murdered inside the consulate, and we assume that he was cut into pieces, they put his body parts in bags, and left."
Turkish media shows videosTurkish media on Wednesday published the identities of 15 members of an alleged hit team sent to kill Khashoggi, including what was said to be a top Saudi forensic official.
Turkish media also released police CCTV surveillance videos said to be of a Saudi "assassination squad." The videos show one of two private jets arriving at Istanbul's Ataturk Airport early in the morning on October 2, the date Khashoggi disappeared after entering the consulate to arrange papers for a marriage.
The CCTV video, which has not been verified, shows nine men going through diplomatic passport control. The video later shows some of the men leaving a hotel near the Saudi consulate and Khashoggi entering the consulate. About two hours later, a black Mercedes van with diplomatic plates leaves the consulate for the Saudi consul general's residence. The 15 men reportedly left Turkey that same evening.
Turkish officials are hoping to enter the consulate and the residence of the consul general to conduct an investigation. Under the Vienna Convention, foreign diplomatic missions are sovereign territory and a host country cannot enter without permission.
Read more: Saudi journalist's disappearance complicates Saudi-Turkish ties
Delicate diplomacyTies between Turkey and Saudi Arabia have been tense over regional political differences, although the kingdom is a major investor in the Turkish economy.
Turkey has backed Qatar in its dispute with Saudi Arabia and its neighbors. The two countries are also at odds over Iran's role in the region and the Muslim Brotherhood, both of which Saudi Arabia views as threats.
Ahmet Kasim Han, politics professor at the Altinbas University in Istanbul, told DW that Saudi Arabia is also sending a message to critical voices that Turkey isn't a safe haven.
"Since the coup attempt on July 15, relations between Turkey and Saudi Arabia have been weakened. That can’t be overstated," Han said. "Even since the Arab Spring (in 2011), Turkey has offered protection to much of the opposition in the Arab world, including the Muslim Brotherhood."
But analysts say that although Turkey is angry over the possible murder of Khashoggi, it also doesn't want to go against Saudi Arabia alone and is looking for political backing from the United States and other allies.
The crisis comes as Turkey's economy is ailing and the country is isolated in the Middle East and among its allies, including the United States over Syria policy and the detention of an American pastor.
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