Al-Qaeda has been planning to use tourist helicopters to attack New York City. From the New York Times, :
As a result, the officials said, security measures for helicopter operators in New York City will be stepped up in a new directive as early as this week. Among the new measures under review is a requirement for operators to conduct airport-style screenings of passengers for suspicious items, said an official with the Department of Homeland Security who had been briefed on the plan. So far, no groundings of helicopter operators are planned.
Personnel at several Manhattan helicopter charter companies said Sunday that although they had already conducted varying degrees of passenger screening themselves, they had heard of no specific safety concerns in recent days from the federal government.
Separately, a senior American intelligence official said that more than 1,000 computer disks had been seized by British authorities during arrests last week of 12 suspected operatives for Al Qaeda in England.