A 19-year-old man has admitted planning a bomb attack, which may have targeted an Elton John concert in Hyde Park on the anniversary of the 9/11 attacks.
Haroon Syed, from Hounslow in west London, was caught after chatting online to undercover British agents.
The Old Bailey heard how Syed tried to buy weapons online - including a bomb vest or explosives - and researched busy areas of London to target.
He will be sentenced in June, after a probation and psychiatric report.
Judge Michael Topolski QC said Syed faced a "discretionary life sentence".
Syed pleaded guilty to a charge of preparation of terrorist acts between April and September last year.
The court heard he had searched the internet for IS [so-called Islamic State], past terrorist attacks, and possible locations - including an Elton John concert on 11 September last year.
He was arrested on 8 September. When police asked for the password to unlock his phone, he replied: "Yeah I.S.I.S - you like that?"
Last June, his brother, Nadir Syed, was jailed for plotting a beheading on Remembrance Sunday in 2014.
Haroon Syed was caught after talking via social media and mobile phone to a fake contact created by the security services.
Syed asked the contact - who was called Abu Yusuf - for a machine gun and an explosive vest.
He said he wanted to "do martyrdom" after causing "damage" with the gun.
Syed later met an officer pretending to be Abu Yusuf in a coffee shop in Slough.
Throughout August, discussions continued between Syed and Abu Yusuf about getting a bomb or a gun.
On 30 August, Syed said: "I might put the bomb in the train and then I'm going to jump out so the bomb explodes on the train... ask the brother if he can make that type of bomb with button."
He 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?"
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".
His defence said he had been groomed by radicals online, but had never intended to carry out an attack.
His lawyer, Mark Summers QC, also argued Syed should have been given help by the Prevent de-radicalisation group - rather than steered to discuss attacks.
Syed entered his guilty plea after an unsuccessful attempt to either get the case thrown out, or exclude key evidence from the online chat.