How does the MB knows what is says it knows?

Egypt’s Brotherhood Still Operates Secretively | TIME.com

(CAIRO) — Egyptian President Mohammed Morsi speaks publicly of firsthand knowledge of a meeting where opponents allegedly plotted against him.

A few months earlier, the most powerful man in his Muslim Brotherhood group, Khairat el-Shater, says he has access to recordings of former military rulers and electoral officials engineering his disqualification from last year’s presidential race.

In Egypt, those statements are seen by security officials, former members of the Islamist group and independent media as strong hints that the Brotherhood might be running its own intelligence-gathering network outside of government security agencies and official channels.

Here's another possible interpretation: intelligence or state security is feeding information to Morsi about the opposition, whether real or made up, with the intention of making him more paranoid and rely on them more. That's one reason why it might not be willing to show its evidence.