CASABLANCA, Morocco, May 18 — Moroccan investigators rounded up and questioned at least 33 suspected Islamic militants Saturday in connection with a series of suicide bombings that killed 41 people, including 13 attackers, in Casablanca, officials said. Authorities are focusing on whether the attackers are linked to a known extremist group that is suspected of ties to the al-Qaida network, a Moroccan official said. The United States said it strongly suspected that al-Qaida had a hand in the blasts, and the White House offered help in the investigation.FIVE GROUPS of attackers — 14 in all — carried out the...