Facebook is facing explosive new questions about when senior executives knew of Cambridge Analytica’s abuse of users’ data, one year on from when the scandal first broke, as federal prosecutors investigate claims that the social media giant has covered up the extent of its relationship with the firm.

The Observer has also learned of claims that a meeting was hosted at the office of Facebook board member and confidant of its CEO Mark Zuckerberg, Marc Andreessen with Christopher Wylie, the Cambridge Analytica whistleblower, in the summer of 2016 just as the data firm started working for the Trump campaign.

Facebook has repeatedly refused to say when its senior executives, including Zuckerberg, learned of how Cambridge Analytica had used harvested data from millions of people across the world to target them with political messages without their consent. But Silicon Valley insiders have told the Observer that Facebook board member Marc Andreessen, the founder of the venture capital firm Andreessen Horowitz and one of the most influential people in Silicon Valley, attended a meeting with Wylie held in Andreessen Horowitz’s office two years before he came forward as a whistleblower.

It marks the end of one of the worst weeks in Facebook’s history. News of more criminal investigations and senior executives leaving the firm was topped by the shocking revelation on Friday that a gunman who killed 49 people at prayer in New Zealand livestreamed the massacre on Facebook. In the hours that followed, Facebook and Google failed to stop the footage going viral with hundreds of thousands of people viewing the video.

But the Observer’s revelations about Facebook open up a new angle on the year-long scandal, raising questions that go right up to board level.

Individuals who attended the meeting with Wylie and Andreessen claim it was set up to learn what Cambridge Analytica was doing with Facebook’s data and how technologists could work to “fix” it. It is unclear in what capacity Andreesen Horowitz hosted and who attened the meeting but it is nonetheless a hugely embarrassing revelation for Facebook, which was revealed last week to be the subject of a criminal investigation into whether it had covered up the extent of its involvement with Cambridge Analytica.

Federal prosecutors in the northern district of California are investigating Facebook’s claims that it didn’t know about Cambridge Analytica’s abuse of Facebook data until it was told by a Guardian reporter, the New York Times reported on Friday. “We are co-operating with investigators and take these probes seriously,” A Facebook spokesman told the New York Times.

Andreessen is one of Silicon Valley’s most influential figures and was an early investor in Facebook. During the period in 2016 in which Facebook said that it was investigating the data abuse by Cambridge Analytica, Christopher Wylie was invited to a meeting at the firm, Andreessen Horowitz.

Wylie, the young Canadian data scientist, would go on to expose the scandal in the Observer a year ago. He revealed then how Cambridge Analytica worked with a Cambridge University academic, Aleksandr Kogan, to harvest Facebook data from users, without their consent, in order to model their personalities and target them politically.

A Silicon Valley technologist with knowledge of the meeting at Andreessen Horowitz said: “There were people who were very concerned by the reports of what Cambridge Analytica was doing with data, and the meeting was set up to try and find out as much about the exploit as possible in order to figure out possible solutions. That’s why Wylie was invited. They wanted his knowledge. He was asked a lot of questions including about the company’s contacts with Russian entities.”

It is understood that a Facebook group was formed, of which Wylie was a member, and it is believed Andreessen remained in touch with Wylie until the Observer broke the story of his involvement in March last year.

A Valley insider who attended the meeting said“The weird thing is that there was no follow-up. The idea was to reverse-engineer the problem to find solutions. But we never learned of any follow-up with Facebook’s security team or any attempt to put the information into action.”

Andreessen Horowitz declined to answer any of the Observer’s questions. Facebook has repeatedly refused to tell members of Congress and the British parliamentary fake news inquiry when senior executives learned of the data abuse. It also declined to answer the Observer’s questions.

Following publication of this article, Marc Andreessen issued the following statement: “The suggestion that I had or hosted a meeting involving Christopher Wylie is flatly and totally untrue. I have never met Wylie in my life. After the election of 2016, a mutual colleague suggested by email that I meet with Wylie, but that meeting never took place. Later, in early 2018, Wylie reached out to me on Twitter and asked for a meeting, which I turned down.”

The company added that it had “no record” of any meeting in its office.

A spokesman said: “Facebook was not aware of the transfer of data from Kogan/GSR [Kogan’s business Global Science Research] to Cambridge Analytica until December 2015. When Facebook learned about Kogan’s breach of Facebook’s data use policies, we took action.”

In another twist, Kogan told the New York Times on Friday that he intends to sue Facebook for defamation for claiming that he deceived the company about how he intended to use the data. A Facebook spokesman described the lawsuit as “frivolous”.

Damian Collins, the chair of parliament’s fake news inquiry, said: “Facebook has declined to say which executives knew what, when. They’ve never explained whether the data was destroyed or where it went.

“Last year on the back of the story breaking, it said it was launching an internal investigation into what other developers had access to data, including companies like Palantir. And it never reported back. It’s hard not to be suspicious about why it is being so evasive.”

David Carroll, a US professor who has pursued Cambridge Analytica through the courts for failing to tell him what personal data it had on him, said it was critical to know who knew what when. “Why is Facebook obfuscating?” he said. “We know Facebook actually employed Joseph Chancellor, Kogan’s business partner before the Guardian’s first report. It defies belief that no one knew anything. The answers Zuckerberg gave to Congress under oath are incredulous. It’s incredible that a year on and we still don’t know these basic facts.”

Carroll’s own ongoing battle with Cambridge Analytica reaches a critical point tomorrow. He is in the high court against Cambridge Analytica’s administrators, to resist the winding up of the companies. Carroll is concerned by how far the administrators concealed the true nature of their relationship with Emerdata, the company’s successor, and the true purpose of the attempt to wind up the company. “They’ve done everything they can to avoid giving me my data. It’s just the same as Facebook is doing. They seem desperate to stop the truth coming out.” The administrators dispute the claims and are pursuing their attempts to liquidate and resisting Professor Carroll’s attempt to remove them.

Ravi Naik, Carroll’s solicitor, who won the Law Society’s award for human rights lawyer of the year for his work on the case, said it was an important moment in his battle to find out the truth about what data Cambridge Analytica held.

“Facebook have tried to avoid telling the truth through these evasive responses to lawmakers. And Cambridge Analytica have tried to do it through insolvency. We know that there was this completely toxic data swamp that was used to target people politically, but we still know nothing about where that data came from and how it was used. We’re still completely in the dark. This is an attempt to crack that vault open.”