"Three of the four suicide bombers who killed 52 people in London last July were known in some form to MI5 but the security service had failed to follow up the leads. Mohammad Sidique Khan had come to the attention of MI5 on five occasions in the previous two years but had never been pursued as a serious suspect. Khan and Shehzad Tanweer had come to the attention of MI5 on several occasions in connection with other inquiries. They were not followed up because they were seen as peripheral figures Jermaine Lindsay’s phone number was also found on MI5 files after July 7, but it was only possible to identify it as his after the attacks in London" (The Times, 19 June 2006).
That analysis, that Khan was just a "peripheral figure", was unfortunate.
The CIA and the FBI could not agree on whether to mount an expensive surveillance operation once he arrived and who should be responsible. So they had no alternative but to put him on the US's no-fly list, thus alerting him to the fact that the US authorities were taking an interest in him. They also informed "British officials". According to Suskind: "British intelligence was certainly told about Khan in March and April 2003". That was over two years before 7/7.