Authorities told ABC News that records show Mohammed Sidique Khan, the eldest of the bombers now believed to be the field commander of the attacks, had called a person who is associated with the Islamic Center, a mosque in Queens, N.Y. Yet, a member of that mosque claimed they had no knowledge of the phone call.

In addition to Khan, two other men linked to the London bombings also had direct ties with the United States.

"Whilst we are watching the ports and the airports trying to prevent people from coming in," said M.J. Gohel, a terrorism analyst at the Asia-Pacific Foundation, "al Qaeda and its global jihadi friends are a step ahead. They have already penetrated into the West and are recruiting Western born Muslims to join terrorism."

Lindsay Germaine, one of the four dead bombers and a Jamaican who left behind a pregnant wife, had recently traveled to see relatives in Ohio.

Furthermore, Magdy El Nashar, 33, who was captured last night at his family's home outside of Cairo and then questioned by British agents, studied at North Carolina State University. Police believe he helped the bombers build their explosive devices. Now they want to know if there are more bombs and would-be bombers.

"It is possible there will be more attempts," admitted Sir Ian Blair, commissioner of the Metropolitan Police. "It is our job to stop them in every way we possibly can. It is more likely now than it was before."

Police said El Nashar, who denied any involvement in the bombings, left England shortly after he rented the house in Leeds, where they discovered a so-called bomb factory. Neighbors in the Cairo suburb told ABC News he arrived there about 10 days before the bombings. They also said they had trouble believing El Nashar could have participated in the attacks.

Now the search continues for another man, whom police believe to be the al Qaeda mastermind of the plot. Authorities said he was on a secondary watch list and believe he crossed the channel by ferry last month in order to elude detection. Hours after the attacks, the man left again by ferry and is thought to be returning to Pakistan, where al Qaeda is known to have bases.

ABC News' Chris Isham, Jill Rackmill, Madeleine Sauer, Rhonda Schwartz, David Scott, Len Tepper, Richard Esposito and ABC News Consultant Alexis Debat contributed to this report.