A report published by Politico on Tuesday revealed that the US has been facing an avalanche of questions from its frustrated allies over the leak of classified Pentagon documents.

How did the leak occur? Who is responsible for it? How did they manage to breach the Pentagon's cybersecurity measures? Those are just some of the numerous questions US officials have been bombarded with.

The leak had left relations with allies in a state of crisis as the incident is reported to be the largest since Wikileaks exposed classified files from 2006 to 2021.

According to a source familiar with the matter, members of the Five Eyes — the intelligence consortium of the US, Canada, UK, Australia, and New Zealand — filed requests for Washington to brief them on the matter. They have yet to receive a response, the source noted. 

They further said that inquiries had been sent separately from the Office of the Director of National Intelligence, the CIA, and the FBI. 

Read more: US takes Pentagon leaks 'very seriously': NSC Spox John Kirby

Other sources told Politico that officials from London, Brussels, Berlin, Dubai, and Kiev requested answers from Washington about the leak. They also asked whether the Biden administration was taking appropriate measures to limit the spread of future intel. 

As of Monday, US officials informed their allies that the investigation is still underway and that they are still determining the full extent of the leak, as per European sources. 

Distress on the part of the Ukrainians has been particularly high because the majority of the documents focused on the conflict in Ukraine. 

"This case showed that the Ukrainians have been absolutely right about that," one of the European sources told Politico. "Americans now owe the Ukrainians. They have to apologize and compensate."

"The manner of the leak and the contents are very unusual," said a former US intel analyst who focused on Russia. "I can’t remember a time when there was this volume of a leak and this broad of a subject matter of authentic information that was just put on social media rather than say, the Snowden files, that went through a group of journalists first."

Read more: Leaked documents may have originated in chatroom for gamers: Reports

As early as March 2, 2023, over 100 intelligence papers were posted on Discord, a social media platform popular with gamers, and appeared to contain classified information on Ukraine, Russian military activity, China, and the Middle East. 

A former US intel source told Politico that the documents contained markings in their corners which suggested that they corresponded to specific wires that were compiled in summary form - a practice is often used by government staffers to prepare briefing packets, he added. 

Although the documents don't appear to have been doctored, one of the papers was identified as heavily altered as it showed significantly higher Ukrainian death tolls.

In a statement on Sunday, Pentagon spokesperson Sabrina Singh said that the White House has assembled an interagency team "focused on assessing the impact these photographed documents could have on US national security and on our allies and partners."

She further confirmed that US officials spoke with “allies and partners" and that the department was still examining the "validity" of the leaks, but it is still unclear who from the Biden administration will be involved in the interagency. 

Kiev senior officials have blamed Russia for the leak, but Kremlin spokesman Dmitry Peskov later said that it is a disease to constantly blame Russia for everything. 

"It is very important to remember that in recent decades, the most successful operations of the Russian special services have been carried out in Photoshop," Andriy Yusov, the representative of the Ukrainian Defense Intelligence Main Directorate, said on Friday, noting that a preliminary examination of documents showed "distorted figures" on losses suffered by both sides in the conflict. 

A senior Ukrainian lawmaker said the leak was "not seen as a big issue here," but on Monday, CNN reported that Ukraine was forced to change some military plans following the leak of the documents that exposed some of the strategies laid down by the Ukrainian military.

The Ukrainian army has already altered some field plans, a source close to Ukrainian President Volodymyr Zelenksy told the news channel without expanding on the details.

The source added that Kiev was not surprised when it was revealed that the United States has been spying on Ukraine, however, there was concern that the leakage even happened.

Read more: US scrambles to find source of leak, evaluates impact on national sec