A man who got off a packed London bus just before it exploded said Tuesday that he had noticed one of his fellow passengers — possibly the bomber — fiddling anxiously with a bag. He said he heard an "excruciating" scream just before the bus blew up.

"This young guy kept diving into this bag or whatever he had in front of his feet," Richard Jones told The Associated Press.

"He must have done that at least every minute if not every 30 seconds. He was getting annoyed, the only reason I noticed it was that he was annoying me," said Jones.

Police said they were investigating the possibility that the four men who attacked the bus and three London subways were homicide bombers. They said personal property belonging to one of the men, believed to be from West Yorkshire (search), in northern England, was found in the wreckage of the bus. That man's family called police to report him missing about 12 hours after the explosion, police said.

Jones, a 61-year-old computer consultant from Bracknell, west of London, said he didn't know if what he saw was significant, but said he had reported it to police. He described the man as being about 6-feet tall, olive-skinned and clean-shaven, wearing tight, light brown trousers and a light brown top.

The No. 30 bus was stuck in heavy traffic in Tavistock Square (search), in the famously literary Bloomsbury neighborhood, on Thursday morning, and Jones said he decided to join a fellow passenger who said he was going to walk instead.

"We banged the back of the bus and the driver then let us off," he said, adding that between 12 and 20 others had joined them. "Immediately, bang, there was an explosion behind me and I turned round and said 'I can't believe it, I just got off that bus'."

"What I did hear just before the explosion was an excruciating scream ... a crazy scream," he said. He could not tell whether the voice was male or female.

The explosion on the bus at 9:47 a.m. killed 13 people. It was about a mile from King's Cross station (search), where police said the suspects had been caught on closed circuit television just before 8:30 a.m.

Jones said a woman on the street asked him if he had just gotten off the destroyed bus. When he said yes, "she actually knelt down in front of me and prayed," he recalled.

He said he hadn't been too shaken by the experience.

"I felt a huge adrenaline rush and then I quickly rationalized it to very much like stepping off the pavement (sidewalk) and almost getting run down by a car, and then stepping back on the pavement and life goes on."