All commercial aircraft in the US will be thoroughly searched within the next day after box cutters, bleach and other suspicious items were discovered aboard two Southwest Airlines planes today.
The FBI, working with the Department of Homeland Security, is investigating how items, which also included clay and matches, got on the planes.
They were discovered by airline employees in plastic bags on two Boeing 737 aircraft on the ground in Houston and New Orleans.
The inspections of nearly 6,000 planes over the next 24 hours are not expected to create flight delays, the Federal Aviation Administration said.
Box cutters were used as a weapon by the September 11 hijackers and have since been banned as carry-on items. Liquid bleach is not permitted on commercial flights, but matches and clay are allowed, according to the homeland security agency's Transportation Security Administration.
FBI director Mr Robert Mueller, speaking to reporters in Houston, said the security breach was under investigation, but is not being treated as a terrorist threat.
"There were no explosives, there was no imminent threat in terms of a capability to commit a terrorist act," he said.