By Jane Dreaper
BBC News health correspondent
It is almost six months since the London bombings - but for many people whose lives were ripped apart on 7 July the full effects are perhaps only now becoming clear.
52 people were killed in the London bombings
Psychological experts believe that about a quarter of those caught up in such horrendous events go on to develop trauma.
But problems may not surface for months or even years.
Nestling among the smart cafes and advertising agencies on a street in the West End of London is an unremarkable building.
Passers by could easily miss the sign saying NHS.
But several storeys up special work is going on, to try to trace several thousand people, who might need help as a result of the London bombings.
Seventy people a month are being referred to this team - and that rate is expected to rise.
A questionnaire has been developed with ten simple questions to try to identify whether someone is struggling.
The clinical psychologists have been in touch with almost 400 people so far and only about 20 have declined any further contact.
Professor Chris Brewin, who leads the team, explains why the approach is low-key.
"In the past there has been a tendency for teams of counsellors to rush in but as time goes on our views about this have changed a lot.
"Really the focus in the immediate aftermath needs to be on very practical things - the time to address the psychological issues is later on."
Professor Brewin said trauma was unusual in that there was often a distinct moment, or period of time, when a person would feel overwhelmed by their thoughts.
These feelings can include helplessness and horror in reaction to the fact that lives were taken by an act of deliberate human design.
Nightmares, flashbacks and a heightened sense of danger are among the symptoms.
"They may find it very difficult to take in what's happening - their sense of time may be speeded up or slowed down - they may feel disconnected from themselves or their environment."
Lucky to survive
David Barnes is a manager with an international mining company based in the West End of London.
He was in the third carriage of the train that was blown up near King's Cross. Like many other survivors, he found it difficult to get back on the Tube.
"It was the first instance that I can recall where I might have died.
"The following week I did come in - I went on the Victoria Line and I had to get off the train - I was uncomfortable and for a couple of weeks I was very suspicious of everybody who got on the Tube. I couldn't really relax that well."
Thanks to the support of friends and family, and his own determination not to let the events disrupt his life, David now feels unscathed.
But he has had an initial session with the NHS Trauma Response team and is keeping in contact with them.
"What was good about it was it was reassuring - I keep telling everyone that I'm fine but I quite like people telling me that I'm fine as well."
Like David, most survivors are getting on with their lives. But those who are traumatised need specialist help.
Dr Patricia d'Ardenne is director of the Institute of Psycho-Trauma, where they are seeing 23 people affected by the bombs.
"A few of the patients that we have seen have been involved in a previous terrorist attack or serious fire in the London area.
"They have coped with these past events extremely well, but, now, after a second assault on them they are now beginning to worry there is some kind of pattern emerging - and that in itself has to be tackled as an irrational belief.
"We go back over the traumatic memory - but we do this in a very safe and graded way.
"We get them to describe things in detail by talking in the first person and in the present tense - this helps them process the memory. We're getting some very good results after only seven or eight visits."
Erville Millar runs mental health services in Camden and Islington. He is co-ordinating the psychological response with other London colleagues.
On the morning of 7 July, he quickly sensed what was unfolding when he heard the dull thud of the bus bomb at Tavistock Square.
"As you can understand by my accent I'm from Northern Ireland and I've seen one or two bombs and some of this brought back those memories to me as well but what encourages me throughout all this is the treatments we offer are hugely successful - they really do work."
This delicate work will continue over the next few years, and is being monitored by the Department of Health.
The aim is to help those who have been affected to return to some kind of normality and help them look forward to things, while recognising that life will never be the same again.