Saudi Arabia Warned US In Writing Last Year About Boston Bomber - Said He Was Going To Attack US City

The Daily Mail did some news reporting beyond tabloid stuff and claim in a new article that Saudi Arabia warned the US, in writing, last year about Tamerlan Tsarnev and that he was going to attempt to bomb a US city. The Saudi's had previously rejected his application for a visa so he could visit Mecca. For them to do that, they must have had some very serious suspicions about the guy which is why they passed them along. If this is true, and the US was warned about him specifically and the FBI was supposedly watching him, then how did he manage to pull off the bombings?