After a second round of attacks in London, British police arrested two young men of Pakistani descent believed to be connected to today's bombings.
One of those captured is said to have been badly beaten by passengers while the other suspect was caught by police after a chase on foot. Eyewitnesses say the arrests were made near the Warren Street and Oval stations.
Law enforcement officials say today's attacks could help their investigation.
"This may represent a significant breakthrough," said Sir Ian Blair, commissioner of the Metropolitan Police.
Police also say they are finding significant clues in the unexploded bombs, including keys as to who built them.
"They've almost certainly recovered some of the explosives intact, and that will tell them a great deal about these explosives' mix," explained Michael Clarke, professor of defense at King's College in London. "They've probably got the detonators."
Today's findings will add to the forensic evidence already gathered over the last two weeks.
The first set of bombers left behind two small, pillbox-sized bombs in their rental car at a train station. Meanwhile, British police discovered large quantities of bomb-making materials at the Leeds apartment now known as the bomb factory. Officials tell ABC News the material used in today's attacks appears to be similar to that used in the homemade explosives two weeks ago.
Police say they also found several sets of fingerprints at the Leeds location besides those of the four bombers in the first attack. Neighbors claim that in addition to those bombers, there were many other men coming and going at strange hours. This all indicates the existence of a larger terror network, police say.
As the investigation continues, those examining the unexploded bombs tell ABC News they appear to be more sophisticated than those used two weeks ago, but that their timing devices failed.
ABC News' Chris Isham, David Scott, and Len Tepper contributed to this report.