WASHINGTON, Feb. 16 - New intelligence information strongly suggests that Al Qaeda has considered infiltrating the United States through the Mexican border, top government officials told Congress on Wednesday. In a wide-ranging assessment of threats to American security, including those posed by Iran and North Korea, the officials also said intelligence indicated that terrorist organizations remained intent on obtaining and using devastating weapons against the United States. "It may only be a matter of time before Al Qaeda or another group attempts to use chemical, biological, radiological and nuclear weapons," Porter J. Goss, the new director of central intelligence, told...