The FSB didn't know about the invasion, because the top brass didn't tell them.

Should have been painfully obvious to anyone in the FSB Ukraine division, given they reportedly surged in size in the months leading up to the war.

> The files show that the FSB unit responsible for Ukraine surged in size in the months leading up to the war and was counting on support from a vast network of paid agents in Ukraine’s security apparatus. Some complied and sabotaged Ukraine’s defenses, officials said, while others appear to have pocketed their FSB payments but balked at doing the Kremlin’s bidding when the fighting started.

https://www.washingtonpost.com/world/interactive/2022/russia...