Osama Bin Laden, chief suspect in the September 11 suicide plane attacks on the United States, may be closer than first thought to developing a crude nuclear weapon.Fear that Bin Laden's al-Qaeda network might be close to developing a dirty bomb was a factor in the United States' decision on Monday to issue a new warning of possible attacks. "US intelligence agencies have recently concluded that Osama Bin Laden and his al-Qaeda terrorist network may have made greater strides than previously thought toward obtaining plans or materials to make a crude radiological weapon that would use conventional explosives to spread radioactivity over a wide area. "The concern is sufficiently deep that some countries have adopted extreme security procedures at their borders, including the increased use of devices that measure radioactivity,"