In a foootnote on page 251 of his 2004 book, Christian Zionism: Roadmap to Armageddon?  Stephen Sizer insinuated that Israel was complicit in the 9/11 terrorist attacks on the USA. (His source is a conspiracy theory in a Scottish tabloid.)
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Why is this antisemitic?
First things first – it is obviously false. It’s a conspiracy theory. 15 of the 16 9/11 hijackers were Saudi nationals. Saudi Arabia is a state that is officially at war with Israel. The USA is Israel’s key ally. Of course Israel was not complicit in the 9/11 attacks.
Rather than being truthful, it is a libel. Is it an anti-Israel libel or an antisemitic one (or both)? Well, throughout the ages, Jews have been accused of being behind everything bad in the world: the Black Death; the French Revolution; the American Revolution; the Russian Revolution; the Great Depression; World War One; World War Two; too much rain; too little rain; England losing on penalties; etc etc. If you were to accuse “the Jews” of being complicit in 9/11, it would undoubtedly be antisemitic. It does not become any less antisemitic if “the Jews” is replaced with “Israel”.
Which is why, unsurprisingly, you normally find this suggestion on racist websites which talk about “Jewish Criminal Power“.
When Stephen Sizer insinuated that Israel was complicit in 9/11, this was not “legitimate criticism of Israel”.
The insinuation was antisemitic.
Stephen Sizer repeated this insinuation in 2015: see here.
 Sizer, S, 2004, Christian Zionism. Road Map to Armageddon, Inter Varsity Press, London