Timeline of US-bound airline security alerts

The discovery of two explosive devices on planes bound for the US has sparked security alarms on both sides of the Atlantic.

The finger of suspicion is being pointed at Islamic extremists, who have already launched a series of plane-based attacks on the US.

Here are some of the details of the plots.

11 September 2001 Attacks

Nineteen men hijacked four commercial airliners in the US, killing nearly 3,000 people when they crashed into the twin towers of New York's World Trade Centre, the Pentagon and a field in Pennsylvania.

The attacks prompted a security overhaul on passenger flights.

2001 Shoe Bomb Plot

British Islamic fundamentalist Richard Reid attempted to blow up an American Airlines flight from Paris to Miami by detonating explosives hidden in his shoes, on 22 December 2001.

He was caught attempting to use a match to light a fuse connected to one of the shoes, was convicted in 2003 and is serving a life sentence at the Supermax prison in Colorado.

2006 Liquid Bomb Plot

In August 2006, three Britons were arrested as they plotted to blow up transatlantic passenger planes with bombs disguised as drinks.

Abdulla Ahmed Ali, Tanvir Hussain, and Assad Sarwar identified seven flights from London's Heathrow airport to San Francisco, Washington, New York, Chicago, Toronto and Montreal.

The plot - the UK's biggest terror investigation, which intelligence officers believe was directed by al-Qaeda figures in Pakistan - prompted a tightening of regulations about liquids on passenger planes which are still in place.

2009 Underpants Attack

Nigerian national Umar Farouk Abdulmutallab is accused of lighting a makeshift bomb smuggled on board in his underwear, as a Northwest Airlines flight with 290 people on board approached Detroit on Christmas Day 2009.

The device fizzled and smoked but did not detonate, and passengers and crew subdued the suspect.

He is on trial in Detroit and has pleaded not guilty.