WASHINGTON, Nov. 19 - Despite having collected substantial information about Iran's nuclear and weapons programs over the last several ears, Western officials have limited intelligence about the crucial question of whether Tehran is trying to meld those two programs to produce a nuclear warhead that can be carried by a missile, administration officials said Friday. The inability to answer that question so far poses an obstacle to the Bush administration's efforts to press for a hard line against the Tehran government. Secretary of State Colin L. Powell said this week that he had seen intelligence indicating that Iran was "working...