The FBI and Homeland Security have issued a nationwide alert about al-Qaeda threats to small airplanes. The bulletin says terrorists have considered renting private planes and loading them with explosives.
Police Commissioner Ray Kelly says there is no specific or credible terrorist threat, but the NYPD has stepped up security as a precaution.
"We're concerned about it," he said. "We of course are concerned particularly because bin Laden spoke about it twice in the notes that were recovered."
Kelly has said that the city will be on heightened alert for the 9/11 anniversary.
Security in lower Manhattan has also been a concern, especially around the September 11 Memorial and Museum.
The lower Manhattan force will eventually rise to 670 — larger than any of the 76 precincts in the five boroughs and entire departments across the country.
Intelligence officials say the biggest threat to US security may be the "lone wolf" terrorist, someone with a single weapon able to carry out wide-scale massacres.
One example is the accused Fort Hood shooter, charged with using two pistols to kill 12 people in a 2009 rampage.
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