The homemade bomb is threatening to replace the gun as America’s protest weapon of choice. And US schools are also experiencing the trend. The high-profile cases of Timothy McVeigh, who has been convicted of the Oklahoma City bombing, and alleged Unabomber Theodore Kaczynski are spawning copycats.

In states as far apart as Georgia and California, police have arrested would-be bombers assembling the material to settle a political or personal grudge.

Meanwhile, in a number of well-publicised cases in the past year, bombs - or more often the threat of them - have repeatedly brought schools to a halt. No national figures are available but schools in one badly hit district in Maryland have been experiencing what Education Week called a “barrage” of bomb threats. They responded to 120 in this school year alone.

Probably the most serious cases to date involve four Texas teenagers who last year set off a series of homemade devices in mailboxes and rubbish cans before being arrested.

Three schoolboys from Syracuse, New York, also received national publicity when they were caught plotting to set off a bomb at their middle school, made from the mixture of fuel oil and fertiliser allegedly used by Mr McVeigh for the bomb that killed 168 people In Oklahoma. Police said the bomb could have destroyed much of the school if successfully detonated.

Both groups apparently picked up their bomb-making recipes on the Internet, where notorious works such as The Anarchists’ Cookbook are on line, containing a chapter entitled “Do You Hate School?” Internet sites reportedly carry directions for manufacturing bombs that can be packed into baby-food jars or envelopes, and have carried postings from European countries like Sweden, Germany and Britain.

Homemade pipe bombs, which have emerged as a favourite among right-wing extremists in the US, have turned up in school lockers across the country.

Bomb threats have a long history as pranks intended to disrupt rather than hurt, and in some cases authorities have over-reacted. When the local bomb squad detonated a suspect device hanging on the door of a Los Angeles school, officers described it as a bomb powerful enough to kill a human being. Local newspapers touted it as the first live explosive device found at a school in living memory.

But by the time three teenaged boys were arrested last month, the threat had been scaled down. The contents of the bag were probably spent air-gun pellets taped together with wires hanging out, a police spokesperson said. The maximum charge facing the three was possession of a simulated explosive device, though all were suspended by their schools.

In Maryland, four crude bombs termed either explosive or incendiary have been found in this school year. No one was hurt, and one boy was arrested with just two plastic tubes stuffed with matchheads in his back-pack. But after a steady string of evacuations, the Prince George’s County school system, with 120, 000 pupils, brought security cameras and extra security staff into the schools, and threatened immediate and lifetime expulsion of any student involved in planting a bomb or making a threat.

School officials also created a video - This is not a Game - that asked parents to look out for unusual chemicals, wires, fuses or tubes at home, and installed a telephone system to trace hoax calls.