The Usual Suspect

The Saudis nailed terrorists who planned to strike Mecca's Grand Mosque.

The article speculates it was ISIL.

But given that Iran-Saudi tensions are high, and given Iran's desire to run the holy sites in Saudi Arabia for all Moslems, I wouldn't be shocked if Iran was behind this. Even if the attackers are Sunni Arabs, Iran has worked with these types.

And the attackers might not even know their true backers.

Pure speculation on my part, of course. But Iran has in the past tried to sow chaos in Mecca.