London bomber Mohammad Sidique Khan featured in a surveillance operation by intelligence services last year, a BBC investigation suggests.
Khan was secretly filmed and recorded speaking to a UK-based terror suspect, according to a well-placed source.
A Radio 4 File on 4 and BBC Two Newsnight investigation also suggests he was in contact with al-Qaeda activists for the last five years.
The Metropolitan Police declined to comment on the investigation.
The programme makers stress there is no independent corroboration that Khan was secretly filmed by intelligence services talking to the terror suspect, who cannot be named for legal reasons.
Khan was a learning mentor at Hillside Primary School in Leeds
But they say that, if correct, it would amount to "a serious failure of intelligence" in the run up to the 7 July bombings.
If true, the new information "would show the intelligence services had him well in their sights but allowed him to slip away", BBC correspondent Richard Watson said.
Until now it had been thought that the plans of Khan, 30, from Dewsbury, West Yorkshire, had remained secret because he and the other bombers had no track record in terrorism and no traces by the intelligence services.
But the BBC investigation suggests that is not the case.
A terror suspect held in connection with the 2002 Bali bombings has alleged that Khan travelled to Malaysia and the Philippines in 2001 to meet leaders of extremist Islamic group Jemaah Islamiah (JI), which is closely linked with al-Qaeda.
The BBC has interviewed academic researcher Dr Rohan Gunaratna who spoke to the Bali suspect after the London bombing.
The suspect said that, after Khan was hosted by notorious JI leader Hambali in Malaysia, he was taken to the Philippines to meet and train with other leaders of the group, suspected of carrying out a number of terror attacks including the Bali bombings of 2002 and 2005.
And in 2003, Khan met with an Islamic extremist in Pakistan who has since confessed to supplying military equipment to al-Qaeda, the BBC has learned.
The extremist, who cannot be named for legal reasons, is a US citizen from a Pakistani family from New York who travelled to Pakistan a week after the 11 September attacks.
Khan and the man also saw each other together in Leeds in 2003, the BBC understands.
"Mohammad Sidique Khan was running a strong cover with his work as a caring teaching assistant in Leeds," BBC correspondent Richard Watson said.
"But a careful study of his background and contacts reveal a number of clues to his extremism which the British intelligence apparently missed."
Viewers and listeners to Tuesday night's programmes will also hear criticism from experts and academics about how radical clerics have "recruited openly" in Britain.
Academic researcher Dr Gunaratna said: "The radical clerics have radicalised young British and European Muslims and have done al-Qaeda's work for them."
And Sir Paul Lever, the former head of the Joint Intelligence Committee, which advises the prime minister on intelligence threats, said there had been a "failure to understand the significance of allowing these clerics to recruit openly".
File on 4 was first broadcast on Radio 4 at 2000 BST on Tuesday 25 October.
It is repeated on Sunday 30 October at 1700 GMT, or you can click here to listen.