Anti-terrorism police investigating the 7 July London bombings say people who knew the attacks were being planned could possibly face prosecution.
Scotland Yard said it continues to be very concerned
Officers at a Scotland Yard briefing also said they continue to be very concerned by the intelligence picture, with 70 investigations continuing.
Some of the intelligence received was described as "very sinister".
The first anniversary of the bombings of London will be marked on Friday by a national two-minute silence.
The silence, at 1200 BST, is part of a day of events to commemorate the attacks and remember the 52 victims.
"The level of counter-terrorist investigations has intensified during the past 12 months," said Peter Clarke, head of the Metropolitan Police's Anti Terrorist Branch.
Officers were "reconstructing" events leading up to the 7 July attacks to identify anyone who might have known they were going to happen, he added.
"It is an immensely complicated piece of work."
"Did anyone encourage them? Did anyone help them with money, accommodation or expertise in bomb making?"
In the days before the attacks their lives had appeared to be a picture of "complete normality", Mr Clarke said.
Their families had been "completely unaware" of what was about to happen, he added.
The investigation remained "enormous", "intensive" and with an "unwavering focus" on finding out the truth, Mr Clarke said.
"We are following many lines of enquiry both here in the UK and overseas."
Officers had taken 13,353 witness statements, there were 29,500 exhibits and more than 6,000 hours of CCTV footage - some of which still needed to be analysed, Mr Clarke added.
"A great deal of progress has been made, but much remains to be done."
At least two of the bombers had visited Pakistan and much of the investigation was focused on trying to find out what they did there, Mr Clarke said.
"The suspicion is they met up with people connected to al-Qaeda and they were attending training camps."
But Mr Clarke added: "The part of the country where these sorts of activities take place is very difficult for the Pakistani authorities and ourselves to get any detailed information about."
Ringleader Mohammad Sidique Khan may well have made his suicide video overseas, Mr Clarke said.
Nail-encased peroxide bombs, similar to hand grenades with nails attached, left by two of the bombers in their car at Luton station could have been used to escape arrest if they had been stopped on the journey to mount their attack, he added.
Officers had disrupted at least three further attacks since 7 July, which would be the subject of criminal trials, Mr Clarke said.
The inevitable secrecy surrounding counter terrorism and legal restrictions on the reporting of impending trials meant the public remained ignorant of the scale of the terrorist threat, he added.
"With terrorist cases taking anything up to two years or longer to come to court it means that the public are unaware of many important things that have happened in this country.
"That is a pity and it means we must guard against blame, recrimination, speculation or myths taking the place of solid public information."
He said the Metropolitan Police Anti-Terrorist Branch had around 70 current investigations spanning London, the UK and the globe.
"The defence of the capital often starts many thousands of miles away. Despite the increase in resources, we are running at or near capacity.
"There is a lot of intelligence to be investigated - some of it is very sinister. It is a very, very concerning intelligence picture."