A teenager from Hertfordshire has admitted making bomb threats to thousands of schools and a United Airlines flight travelling from the UK to San Francisco last month.

George Duke-Cohan was remanded in custody on Monday after pleading guilty at Luton Magistrates Court to three counts of making hoax bomb threats.

The 19-year-old sent bomb threats that resulted in over 400 schools being evacuated in March 2018.

Duke-Cohan was arrested just days later, but the following month, while still under investigation, he sent a mass email to schools in the UK and US claiming that pipe bombs had been planted on the premises.

On 9 August, the hacker group known as ‘Apophis Squad’ claimed on Twitter that flight UAL 949 had been grounded due to their actions, but it was in fact a threat by Duke-Cohan who was then on pre-charge bail for the school threats.

The teenager had called in threats to the United Airlines flight via San Francisco Airport and their Bureau police.

In a recording of one of the phone calls which was made while the plane was in the air, the National Crime Agency (NCA) said Duke-Cohan “takes on the persona of a worried father and claims his daughter contacted him from the flight to say it had been hijacked by gunmen, one of whom had a bomb”. 

The NCA released a recording of the call on Tuesday following Duke-Cohan’s court appearance. 

On arrival in San Francisco the plane was the “subject of a significant security operation” in a quarantined area of the airport and all 295 passengers had to remain on board. This caused disruption to onward journeys and financial loss to the airline, the NCA said. 

In an operation supported by Hertfordshire Police, Duke-Cohan was arrested by NCA officers for the third time at his home in Watford on Friday 31 August.

Officers recovered multiple electronic devices belonging to him, the use of which was in contravention to the pre-charge bail conditions imposed on him, the NCA said. 

NCA Senior Investigating Officer Marc Horsfall said Duke-Cohan had caused  “serious worry and inconvenience to thousands of people, not least an international airline”.

Horsfall added: “He carried out these threats hidden behind a computer screen for his own enjoyment, with no consideration for the effect he was having on others.

“Despite being arrested and having conditions imposed restricting his use of technology, he persistently broke those conditions to continue his wave of violent threats.”

Horsfall said authorities take bomb threats “extremely seriously” and warned potential offenders that operating online does not offer anonymity.

“We will identify you and you will be brought before the courts,” he said. 

Duke-Cohan is due to appear at Luton Crown Court on 21 September.