The leader of the British Labour Party, Jeremy Corbyn, has admitted he was once a member of a secret Facebook group in which people discussed anti-Semitic conspiracy theories about Holocaust denial and Jewish involvement in the 9/11 attacks—but he insists he never saw the offensive messages.

A detailed dossier by businessman and activist David Collier, who says his mission is to “expose lies and anti-Semitism,” showed that Corbyn posted several times in the “Palestine Live” group after being tagged by other members, but that he left around the time he was elected Labour leader in 2015.

Screenshots show members openly using terms such as “ZioNazi” and “JewNazi,” debating whether U.K. national broadcaster the BBC is controlled by Zionists, and one member said he was reading Mein Kampf, commenting: “Everybody should be forced to read it, especially Jews who have their own agenda as to why they were not liked.”

Corbyn allegedly replied to comments in threads where the term “Zio” was used and Collier’s dossier claimed that prominent members of the Labour Party—including Members of Parliament Clive Lewis and Chris Williamson, and Corbyn’s son and senior Labour adviser Sebastian—were also members of the group.

When confronted about his membership of the group by the Press Association, Corbyn said he only commented to reply to people who had tagged him such as with “a suggestion on the vote on recognizing Palestine, which I supported, and inviting a doctor to speak at an event.”

He added: “Had I seen [the anti-Semitic post], of course, I would have challenged it straight away, but I actually don’t spend all my time reading social media. I have never trawled through the whole group. I have never read all the messages on it. I have removed myself from it."

Corbyn’s right-hand man and Shadow Chancellor John McDonnell MP added confusion to the timeline of Corbyn’s membership in the group on Friday by contradicting the claim that he never saw the slurs. McDonnell said Corbyn left the group “when he discovered, and it was pointed out to him that there were some people in it expressing anti-Semitic views.”

Labour has repeatedly been accused of failing to deal with anti-Semitism within the party since Corbyn became leader. An internal investigation denied the party was “over-run” with anti-Semitism, but admitted there was an “occasionally toxic atmosphere.”

High-profile incidents have included former London Mayor Ken Livingstone being suspended, but not expelled, for claiming that Hitler was a Zionist. A member of parliament was suspended for suggesting Israel should be relocated to America. Jackie Walker, the former vice chair of pro-Corbyn pressure group, Momentum, criticized Holocaust Memorial Day for only commemorating Jewish victims.

According to Collier, Walker was a member of the secret Facebook group and in one post is seen to ask: “How safe is this group?”, to which the group’s alleged founder replied: “Very. No one is allowed in who is not trusted. I am very very careful, it is a secret group.”

Joseph D. Glasman, head of political and government investigations at Campaign Against Anti-Semitism, told the Jewish Press: “There is no conceivable justification for Jeremy Corbyn’s participation in this group. One of Mr. Corbyn’s slogans is ‘Standing up, not standing by,’ but in this case he has not stood up but instead he actively joined in.”

He added: “As a result of these allegations, we will be filing a disciplinary complaint to the Labour Party against Jeremy Corbyn, Clive Lewis [Labour MP], [and Labour staffers] Sebastian Corbyn, and Jack Bond.”

A spokesperson for Corbyn said: “I don’t think anyone is suggesting that anything that Jeremy has written in any Facebook group or anywhere else constitutes anti-Semitic comments. As I understand it, there were thousands of members of this group. As anyone knows, in social media all sorts of things are posted which often others participating aren’t even aware of.”

The spokesperson added: “It is repugnant if there are anti-Semitic posts—and I gather there are. If they involve anyone to do with the Labour Party, then investigation and disciplinary action will be taken.”