The bombs that tore through a crowd of spectators at the Boston Marathon could have cost as little as $100 to build and were made of the most ordinary ingredients — so ordinary, in fact, that investigators could face a gargantuan challenge in attempting to use bomb forensics to find the culprit.

Investigators revealed Tuesday that fragments recovered at the blast scene suggest a simple design: a common pressure cooker of the kind found at most discount stores, packed with an explosive and armed with a simple detonator. A final ingredient — a few handfuls of BBs, nails and pellets — helped ensure widespread casualties when the two devices exploded Monday near the race’s finish line, law enforcement officials said.

The devices’ design was immediately recognized by counterterrorism experts as a type touted by al-Qaeda for use by its operatives around the world. Similar devices have been used by terrorists in mass-casualty bombings in numerous countries, from the Middle East to South Asia to North Africa.

Yet the bomb’s simplicity and garden-variety ingredients complicate the task of determining whether the maker was an international terrorist, a homegrown extremist or a local citizen with a grudge, investigators and experts say.

“This is going to take a very long time,” said a federal law enforcement official involved in examining the deadliest bombing on U.S. soil in nearly two decades. The official spoke on the condition of anonymity to discuss an active investigation.

The FBI agent leading the inquiry, Richard DesLauriers, said as much at a news conference Tuesday. “We are doing this methodically, carefully, yet with a sense of urgency,” he said.

The simple bomb design could imply that the maker was an amateur, incapable of acquiring more sophisticated materials, veteran investigators and forensics experts said. But they said it also could be the work of a sophisticated bomb maker taking great care to cover his tracks.

“He might deliberately choose to use a less sophisticated device because he knows the explosives will be harder to trace,” said Robert Liscouski, a former homeland security assistant secretary now with Implant Sciences, a company that makes bomb-detection devices. “It’s what you would expect of someone who wants to carry out more of these attacks.”

(Photos: Aftermath of the attacks)

What appeared certain was that the bombs unleashed a relatively modest explosive force, compared with more lethal improvised explosive devices or suicide bombs in Iraq or Afghanistan. Yet the twin explosions were powerful enough to shatter windows and sever the limbs of multiple bystanders who were close to them.

The fact that the bombs were not powerful enough to gouge craters in the sidewalk or inflict structural damage on nearby buildings suggested to some investigators that they had a common explosive such as black gunpowder, rather than something like plastic explosive. Black powder is widely sold at sporting goods and discount stores.

DesLauriers said pieces of the bombs and residue have been sent for testing at the bureau’s laboratory at Quantico.

“The black powder, or smokeless powder, you can buy in Wal-Mart or gun shops. It’s used for hobbies,” said Michael Bouchard, a former assistant director of the Bureau of Alcohol, Tobacco, Firearms and Explosives. “All you need is a fuel and an oxidizer, and the Internet is great for explaining how to do this.”

Bouchard said the shock wave and hail of shrapnel might have caused more casualties, except for the fact that the bombs were probably placed on the ground and surrounded by tightly packed crowds of spectators who absorbed the worst of the impact.

“There were so many people around it, they took the brunt of the blast,” Bouchard said. “They shielded the runners.”

Other experts said they suspect that a single person or small group was probably behind the attack, in part because there has been no claim of responsibility. Several international terrorist groups, including the Pakistani Taliban, have so far disavowed having any role in Monday’s bombing.

“The location of the explosive devices in the crowd at the finish line indicates that the intent was to cause mass casualties rather than targeting specific individuals or assets,” said Alexia Ash, head of North America forecasting for Exclusive Analysis, a security consulting firm. Ash said the culprit clearly achieved “effective operational security. . . as well as the capability to construct several small but effective IEDs.”

Other clues to the design of the bombs emerged when emergency room physicians in Boston confirmed that many victims suffered shrapnel wounds. Some who were standing near the bombs were struck dozens of times by what appeared to be BB-like pellets and carpenter’s nails.

Federal investigators confirmed that the shrapnel had been inside pressure-cooker devices. They said fragments of black nylon indicated at least one of the bombs was concealed in a bag or backpack.

The pressure-cooker bomb is a standard part of al-Qaeda’s repertoire and has been hailed as cheap and effective in articles and handbooks written by veterans of the terrorist group. In 2010, the al-Qaeda-linked publication Inspire offered instructions for building a pressure-cooker bomb in an article titled, “Making a bomb in the kitchen of your mom.”

Faisal Shahzad, a 31-year-old Pakistani American living in Connecticut, used a pressure cooker as part of a bomb that he tried to set off inside a vehicle in New York’s Times Square on May 1, 2010. He had trained with extremists in Pakistan and was sentenced to life in prison.

In 2006, in one of the deadliest attacks in recent years in India, terrorists planted similar devices on trains along Mumbai’s regional rail network, killing 209 people and wounding more than 700 others. The lethality of the devices has prompted warnings by U.S. agencies urging vigilance in watching for bombs concealed inside the ubiquitous aluminum pots.

“The presence of a pressure cooker in an unusual location, such as a building lobby or busy street corner should be treated as suspicious,” warned a Homeland Security brochure issued in 2010.