Five American students have been arrested and charged with planning a Columbine-like assault on their high school in a plot that was foiled not by increased awareness or security, but by a 17-year-old girl who knew of the plot and warned a favourite teacher.

The girl, Amylee Bowman, who had agreed to smuggle some of the guns into the school, was charged with conspiracy to commit murder.

Two 15-year-olds, a 16-year-old and a 17-year-old were also charged with conspiracy to commit murder, and crimes including possession of ammunition.

The students allegedly planned to set off a bomb inside their high school in New Bedford, a coastal New England fishing city, shoot their fellow students and teachers as they fled, then go to the roof and commit suicide together.

The plot was eerily similar to the 1999 Columbine high school massacre in Littleton, Colorado, when 15 people died, including the two gunmen who committed suicide.

Like countless other American schools, New Bedford High reacted to the Columbine killings by posting police officers on its campus, and urging its students to come forward if they learned about such schemes. The school has an enrolment of 3,250.

Yet apart from the warning by Ms Bowman to her teacher, few of those other steps apparently had much effect - including several clues that were evident well in advance of the arrests.

Court records show that the five students involved in the plan were self-described “freaks” who wore black trenchcoats (the uniform of the Columbine killers ); complained of being picked on because of their dress; and spoke of taking revenge against athletes and other more popular classmates.

One kept satanic masks, a meat cleaver, an axe, instructions for making a bomb, and a photograph of Adolf Hitler in his bedroom, where police also discovered spent cartridges and torture devices. A janitor also discovered a note promising that “jocks, thugs, preps and faculty” would be killed.

Parents seemed less than reassured after the plotters were eventually arrested. Dozens of police and five bomb-sniffing dogs searched the school, but more than half the students stayed home on the first day that classes resumed.

Superintendent Joseph S Silva Jr said that in the light of the incident, security measures would be reviewed.