The bombs were identical in size and design: one in a black nylon duffel bag, the other in a black knapsack with grayish trim. Each was made of 10 kilos of powerful commercial explosives - essentially sawdust soaked in nitroglycerine - with nuts and bolts for shrapnel. Left on seats in a commuter rail car, they were detonated almost simultaneously using cell phones.
The explosions ripped into the steel, plastic and fiberglass, sending a deadly shock wave through the passenger compartment and hurling twisted chunks of metal, debris and tiny bits of the bombs hundreds of feet in every direction. Almost everyone in the rail car should have been killed.
But the bomb maker, like those who worked with him that day, had assembled the explosives to help catch terrorists. An expert bomb investigator, he was helping to train his peers from across the country.
The blast, which took place early this year at the China Lake Naval Air Weapons Center in the California desert north of Los Angeles, was observed by dozens of state, local and federal investigators, including the commander of the New York Police Department Bomb Squad.
It was part of a highly specialized FBI training seminar, a five-day program that is presented several times each year across the country and includes the detonation of a thousand-pound truck bomb.
The program helps bomb investigators better understand the awesome power of such devices and, according to the chief trainer, Special Agent Kevin Miles, it teaches them how to piece together a device after an enormous blast and reveal a bomb maker's unique signature. It gives them the confidence to know that they can do their job, he said, to help find the bomber.
The class, the Large Vehicle Bomb Post-Blast School, is one of more than a half-dozen ways that big-city police departments and federal investigators keep up with the latest in terrorist technology in a deadly and fast-moving discipline.
Others include routine training, such as that conducted by bomb squad investigators in New York City, who build bombs their colleagues must defuse.
There is an FBI-run database that, with help from the Pentagon and the CIA, tracks emerging bomb technologies and trends from Iraq and Israel to Europe and South America.
And the federal Bureau of Alcohol, Tobacco, Firearms and Explosives is close to completing a sophisticated computer program that can use such data as the size of a bomb crater and the distance that debris was propelled by a blast to help solve terrorist bombings.
"The only thing limiting a bomb maker is the bomber's imagination," said Lt. Mark Torre, the New York Bomb Squad commander who attended the vehicle bomb school this year.
Torre, who has spent more than half of his nearly 20 years in the department investigating bombers, said that as the bombers' technology improved, so did the sophistication of the men and women charged with stopping and catching them.
"It's not a race; it's more like a marathon," said the lieutenant, an intense man who, while animated, seems nonetheless to possess an unnerving sense of calm and a placid stare.
The NYPD Bomb Squad, the nation's oldest, largest and very likely the busiest, celebrated its 100th anniversary in 2003, tracing its roots back to a turn-of-the-century unit that investigated extortion bombings against Italian immigrants.
The detectives, police officers and supervisors assigned to it work and train almost nonstop, Torre said. But they are cautious about discussing the work, citing the extreme danger that they - and, to a lesser degree, the public - face every day, and they are reluctant to reveal what they do and how they do it for fear of aiding their adversaries.
Largely for those reasons, the close-knit community of bomb squad investigators and technicians across the country is something of a secret society.
So there is much Torre will not discuss, including how many men and women are assigned to the unit. But he acknowledges that the number has grown substantially since Sept. 11, 2001, along with the workload.
Before the Sept. 11 attacks, the annual number of reports of suspicious packages investigated by the NYPD numbered in the hundreds, although the bomb squad was not needed in every instance, officials said. This year, it has already handled more than 7,000, with 1,848 in just the three-week period after the July 7 bombings in London.
Police Commissioner Raymond W. Kelly said in an interview that gathering intelligence on terrorist bombings around the world and determining how that information can apply in New York was an essential element of the bomb squad's work. "One of the concerns," he said, "is the improvement in bomb-making trade craft that we see in Iraq."
To that end, Kelly has underwritten extensive training and travel for bomb squad investigators to Israel, Britain and elsewhere to learn first-hand about how terrorists in those nations are building bombs and carrying out attacks.
An important training exercise, Torre said, is the squad's practice of building and defusing - rendering safe, in the parlance of bomb technicians - dummy devices, from suitcase and backpack bombs to almost everything imaginable, with firing mechanisms from simple to highly complex.
One investigator takes on the role of bomb builder, constructing a device without explosives, while another seeks to figure out how the bomb works, using X-ray equipment and other technologies, and render it safe.
"It's important because it keeps you sharp, it tests your skills in the various disciplines that we utilize," Torre said, "whether it be an investigative skill as in X-ray interpretation or a ... bomb disabling tool."
The dummy devices have a real firing system, usually setting off an electronic firing mechanism if the bomb technician fails. Torre would not offer his teams' success rates, saying only, "We win many more than we lose."