ISLAMABAD, Pakistan, March 1 — Khalid Shaikh Mohammed, suspected of planning the Sept. 11 attacks on New York and Washington and one of the F.B.I.'s most wanted terrorists, was detained by Pakistani authorities this morning and is now in American custody, officials said. {snip} Officials suspect that Mr. Mohammed was also involved in the simultaneous bombings of the American Embassies in Kenya and Tanzania in 1998, as well as the bombing of the United States destroyer Cole in Yemen in October 2000.