A teenager was snared by secret agents as he plotted a bomb attack, possibly targeting an Elton John concert in Hyde Park on the anniversary of the 9/11 attacks.
Haroon Syed, 19, from Hounslow, west London, pleaded guilty on Thursday at the Old Bailey to a charge of preparation of terrorist acts between April and September last year after a last-ditch attempt to get his case thrown out failed.
The court heard how Syed tried to get weapons online, including a bomb vest or explosives, and trawled the web to find a busy area in the capital such as Oxford Circus to launch a mass casualty attack.
Syed was caught chatting online with officers from the British Security Service who posed as a fellow extremist who could help him source the weapons.
Judge Michael Topolski QC adjourned sentencing until June 8 for a probation and psychiatric report to be done. He said Syed faced a "discretionary life sentence".
As Syed was being drawn in by agents, his older brother was jailed for plotting an Islamic State-inspired Poppy Day attack.
At an earlier hearing, the court heard how key evidence was gathered from Syed's communications with the fake contact Abu Yusuf via mobile phone and social media.
Syed asked for "gear" for his "opp" and when asked to give details, he said he needed a machine gun and an explosive vest "so after some damage with machine gun do martyrdom ... that's what I'm planning to do."
When Abu Yusuf said it would be costly, the defendant told him he was "broke" but pressed on, saying: "You have to find out the price for the machine gun, any gun."
Initially Syed cried off meeting his contact, complaining that he was being followed by police who will "have eyes on me if I come".
An officer pretended to be Abu Yusuf when they did finally get together at a Costa Coffee in Slough and their conversation was taped.
Throughout August, the discussions continued about making or getting a bomb and acquiring a gun, even though Syed confessed he had never used one before.
On August 30, Syed stressed that he needed a "portable" device, saying: "I might put the bomb in the train and then I'm going to jump out so the bomb explodes on the train ... So ask the brother if he can make that type of bomb with button."
Meanwhile, he researched targets and they arranged to pick up the bomb in exchange for £150 the following week.
Syed asked Abu Yusuf to make sure there were lots of nails in it and added: "I was thinking of Oxford Street ... If I go to prison, I go to prison. If I die, I die, you understand."
He searched the internet for IS, past terrorist attacks, and potential locations which included an Elton John concert in Hyde Park on September 11 last year.
Police moved in to arrest the defendant at his home on September 8 and his phone was seized. Asked for the password to unlock the device, Syed said: "Yeah I.S.I.S - you like that?"
In legal papers prepared for his defence, Syed was described as "highly vulnerable due to family history, lack of education, addiction to violent online games and the arrest and imprisonment of his brother".
He was groomed by radicals online but he never intended to carry out an attack as his chats with the agent were a "fantasy to see how far it would go", according to his defence statement.
Syed entered his guilty plea in the wake of an unsuccessful attempt to either get the case thrown out, or exclude the key evidence from the online chat.
His lawyer Mark Summers QC argued he should have been given help by the Prevent de-radicalisation group rather than steered in the opposite direction.
He said: "The proper response of the State should have been to engage Prevent to help this young man, to steer him away from the path it was feared he was going down, rather than guiding him down it."
But the judge refused Mr Summers' application and also rejected a defence request to offer an indication of what Syed's likely sentence would be if he admitted the offence.
Last June, IS-inspired Nadir Syed, 23, also from Hounslow, was jailed for life with a minimum term of 15 years after he was found guilty of plotting to carry out a Lee Rigby-style beheading around Remembrance Sunday in 2014.