It said he was planning to target US economic interests including gas/oil pipelines, power generating dams and strike ships or oil tankers by using explosive-laden speed boats in international waters.
Some background on al-Mauritani can be found in this article, coincidentally published in today's edition of Spiegel Online.
ISPR also mentioned that the operation to arrest al-Mauritani - along with Abdul Ghaffar al-Shami (aka Bachar Chama) and Messara al-Shami (Mujahid Amino) - was "planned and conducted with technical assistance of United States intelligence agencies with whom Inter Services Intelligence has a strong, historic intelligence relationship".
True, but in recent months that relationship has been in tatters and is still an open wound. Giving the Pakistanis live information on important al-Qaeda suspects and allowing them to make all the running in the arrests would be a nice gesture, just to show that there are no hard feelings. Wouldn't it?