LEEDS, England -- Each weekday morning at 8, Mohammad Sidique Khan used to visit the home of Deborah Quick to pick up her two daughters, Harley and Robyn, and take them to Hillside Primary School.

The two girls were members of what Mr. Khan called his "breakfast club," an early morning service to help parents on welfare get their kids to school in time for an 8:30 breakfast and 9 a.m. start. Conscientious and cheery, Mr. Khan was "brilliant," recalls the girls' mother.

As word spread of his kindness, other parents in Beeston, a deprived and drug-blighted district of this northern English town, asked Mr. Khan to pick up their children, too. Unable to fit them all into his small, navy blue Vauxhall Corsa, Mr. Khan started walking them to school. He continued to take Ms. Quick's daughters, then 6 and 4, to school until early last year.

About 18 months later, around the same time of day on July 7, Mr. Khan and three friends boarded a train from Luton, 30 miles north of London, then fanned out on three subways and a red double-decker bus. They then triggered bombs in their backpacks, killing at least 56 people, including themselves, and injuring about 700, police say.

To many in Beeston, Mr. Khan, 30 years old, had seemed to be a good citizen. He was the exact opposite of terrorist recruits who keep a low profile, in that he was a homegrown terrorist hiding in plain sight and thus much harder for authorities to identify.

Since July 7, London has been on the highest terrorist alert level. Thursday, four small explosions hit three subway trains and a bus in attacks that resembled the July 7 bombings. There was one injury but no fatalities, police said.

The eldest of the four suspected suicide bombers by at least eight years, Mr. Khan appears to have been a leader of the small group, although investigators continued their search this week for others involved in the plot. He traveled with one of the other suspected bombers, Shehzad Tanweer, 22, to Pakistan for three months, arriving in Karachi last November from Istanbul and leaving in February. Mr. Khan and Mr. Tanweer also shared a passion for kickboxing. Mr. Khan taught the sport at a local community center.

In Beeston, family, friends and neighbors are now scouring their memories for signs of trouble they missed. Mr. Khan's family, in a statement issued through police, says he must have been "brainwashed" into becoming a suicide bomber because "we know him as a kind and caring member of our family."

Ms. Quick's live-in boyfriend, Sonny Dan Lowther, 25, says he briefly glimpsed two sides of Mr. Khan. One day after school, says Mr. Lowther, Mr. Khan urged him to take up kickboxing as a way to curb his "negative energies." Mr. Lowther says he declined and was surprised at Mr. Khan's agitated reaction. "I've seen an ... angry side to him that made me uncomfortable," said Mr. Lowther, who describes himself as a part-time rapper, recovering drug addict and failed bicycle repairman.

Others saw brief flashes of anger but little else to arouse suspicion. In addition to being a teacher's assistant who worked well with kids, Mr. Khan won respect as a social worker committed to ridding the streets of drugs, co-authoring a 2001 government study on fighting drug use. He was invited to the home of Hillside Primary's head teacher, Sarah Balfour, whose husband, Jon Trickett, is a member of parliament. On a trip to London in July last year, Mr. Khan and his students got a tour of parliament from Mr. Trickett.

Since the attacks, police have also been looking for missed clues that might have helped them spot Mr. Khan's drift into extremism. Mr. Khan's name, it turns out, had come up during investigations into at least one past terrorist plot -- a thwarted spring 2004 plan to explode a truck bomb in London, according to investigators. A Pakistani-American in police custody in New York, who has pleaded guilty to providing material support to terrorists, including al Qaeda, also recognized Mr. Khan's photograph when it was shown to him last week, according to an official at the Federal Bureau of Investigation.

Born in Leeds to immigrant parents from Pakistan, Mr. Khan grew up in Beeston and attended a school in south Leeds. A former colleague says he met his wife, Hasina, a Muslim whose parents are of Indian descent and moved to Britain from South Africa, at Leeds Metropolitan University. The university says Mr. Khan's wife attended the college, but it has no record of Mr. Khan himself taking courses.

In the late 1990s, Mr. Khan got a part-time job with a local government agency working with troubled youth in Beeston, where unemployment had soared to 40 percent among men of Pakistani and Bangladeshi descent. During summer holidays he ran workshops for kids.

Afzal Choudhry, a community worker who took part in the summer sessions, praises him for being "always ready to get involved." Mr. Khan, he says, was not particularly religious when they first met around 1997. Mr. Khan sometimes got "what we call the Friday feeling" and would go to mosque for Friday prayers, but he otherwise didn't pray much, says Mr. Choudhry.

He says that Mr. Khan later started praying five times a day and, along with many other young Muslims in the area, began to show interest in fringe Islamic preachers who visited Leeds. One of them, a Jamaican-born cleric named Abdullah el-Faisal, spoke twice at mosques in Beeston. Mr. Faisal, who was sentenced in 2003 by a British court to nine years in prison for urging violence and murder, sometimes worked with Abu Hamza al-Masri, a vituperative London preacher who was arrested in May last year and now faces charges in London of inciting murder and racial hatred.

Mr. Khan was also a regular visitor at Iqra, a tiny Islamic bookshop set up in Beeston about five years ago, according to several neighbors. It started out promoting moderate Islam and study of the Quran, but it gradually took on a more radical tinge, according to people who have visited the store. After the London bombings, forensic police officers in white protective suits and blue rubber gloves searched the store for six days.

While flirting with more radical forms of Islam, Mr. Khan also moved further into Beeston's mainstream, taking a full-time job as a teaching assistant at Hillside Primary School. He also accepted an offer from Mr. Choudhry to work on a government-funded study of Beeston's drug problems.

In a note appended to the 2001 report, Mr. Khan wrote: "I have tried to support and help local drug users in the past, but at best this has been haphazard. I feel that the knowledge and experience on drugs and community research that I've gained through the training and the field work has been invaluable."

As part of his duties at Hillside Primary School, Mr. Khan started his breakfast club, taking Ms. Quick's daughters, Harley and Robyn, to school and carefully observing their development. In a detailed report in November 2003, written for government social services, he said, "They are both chatty and will talk to me as well as other children on the way down to school. Harley has a stronger character and will look after Robyn's needs."

Mr. Khan stopped taking Harley and Robyn to school in January 2004, when they went to live with their grandmother.

Though usually preoccupied by local affairs, Mr. Khan strongly opposed Britain's joining American forces in the 2003 invasion of Iraq. "You could not carry out a civilized conversation with him on Iraq," said Arshad Chaudhry, head of the Leeds Muslim Forum, an umbrella group of local Islamic leaders. But his anger aroused no suspicion: Many others, Muslim and otherwise, vehemently opposed the conflict, too.

Late last year, Mr. Khan prepared to move into a new house in a tidy suburb of Dewsbury, a town 10 miles southwest of Leeds. He rented the two-bedroom home from the local council and started to redecorate.

"I thought he was a really nice guy and thought 'He'll be good for our neighborhood,' " said Imran Zaman, who lives two doors away. Soon afterward, Mr. Khan disappeared, said Mr. Zaman.

In November last year, Mr. Khan took a leave of absence from Hillside Primary and flew to Pakistan with his Beeston friend, Mr. Tanweer. A few weeks later, he wrote a resignation letter to his school. After returning from Pakistan in February, he returned to his new home. His neighbor, Mr. Zaman, who says he didn't know Mr. Khan had been in Pakistan, says he last saw Mr. Khan as he jumped into a red Mercedes-Benz on the evening of July 6, the day before the bombings.