The U.S. on Tuesday joined foreign ministers of the G-7 economic powerhouse nations to condemn "in the strongest possible terms" the slaying of journalist Jamal Khashoggi in the Saudi Consulate in Istanbul.

"Those responsible for the killing must be held to account," the G-7 ministers said in a joint statement. "Saudi Arabia must put in place measures to ensure something like this can never happen again."

It was not immediately clear what measures the group wants to see from Saudi Arabia. But the United States' attachment to the letter offers one of the most direct criticisms yet from President Donald Trump, who has previously weighed the consequences of the killing against his strong alliance with the kingdom.

Khashoggi, a columnist for The Washington Post and critic of the royal family, entered the consulate on Oct. 2, where he was killed by a crew of Saudis. The Saudi government spent days denying that Khashoggi had been killed, as Turkish officials had claimed.

But on Friday, Saudi Foreign Minister Adel al-Jubeir said the slaying was a "tremendous mistake." He denied that Crown Prince Mohammed bin Salman had ordered the killing.

Other reports, however, have cast doubt on claims that the Khashoggi's death was accidental. For instance, CNN reported that one of the men involved in the killing wore Khashoggi's clothes out of the consulate after he had been killed, fueling speculation that Saudi Arabia was preparing an alibi to back up their original claim that Khashoggi left the building shortly after he arrived.

"The explanations offered leave many questions unanswered," the G-7 said in the statement. The group is composed of Canada, France, Germany, Italy, Japan, the United Kingdom, the U.S. and the European Union.

Trump had expressed his reluctance to call out Saudi Arabia earlier this month, even as lawmakers in his party came to conclusions as the circumstantial evidence mounted. "Here we go again with you're guilty until proven innocent," Trump said last week in an interview with The Associated Press.

The president has also had his differences with the G-7. On the eve of the latest G-7 Summit in Quebec, Trump took shots from his Twitter account at French President Emmanuel Macron and Canadian Prime Minister Justin Trudeau on trade issues.

Trump tweet

But Trump relented, saying last week that it "certainly looks" as though Khashoggi was dead and warning of "very severe" consequences if Saudi leaders were found responsible.

The G-7 statement called for "a thorough, credible, transparent, and prompt investigation by Saudi Arabia, in full collaboration with the Turkish authorities, and a full and rigorous accounting of the circumstances surrounding Mr. Khashoggi's death."

"The circumstances of Mr. Khashoggi's death reaffirm the need to protect journalists and freedom of expression around the world," the G-7 ministers said. "We also extend our deepest condolences to Mr. Khashoggi's family, his fiancee, and his friends."