Because of their destructive nature, bomb blasts leave behind twisted metal and charred fragments that appear challenging for investigators looking for clues.
The hunt for the Unabomber, for example, took 18 years before the culprit was caught. On other occasions, however, forensic clues or careless blunders by the bombers led to their identification within days.
Here is how chance and technical savvy helped investigators solve past high-profile cases.
THE UNABOM CASE
From May, 1978, when he first struck, until April, 1995, when his last bomb was delivered, a mysterious technology-hating man sent explosive parcels that killed three people and injured 24 across the United States.
Code-named UNABOM (UNiversity and Airline BOMber), the investigation grew to involve more than 150 people who unsuccessfully examined thousands of leads over the years. The turning point came in 1995 when the bomber sent the FBI a 35,000-word manifesto that he wanted to make public.
After furious internal arguments, the FBI agreed, hoping it would elicit tips from the public. The Washington Post published the document that September. When David Kaczynski, a social worker in upstate New York, saw the bomber's screed, it reinforced the misgivings that he and his wife had long shared about his estranged brother, Theodore.
Those suspicions came amid hundreds of other tips but David Kaczynski provided the FBI with letters written by his brother, which were analyzed by a psychological expert. With that data and a cross-reference to the suspect's past travels, the FBI was able to get a warrant to search the forest cabin where Theodore Kaczynski lived as a recluse. Agents found shelves crammed with bomb-making chemicals and he was arrested. He was sentenced in 1996 to four life terms.
THE FIRST WORLD TRADE CENTRE BOMBING
A terrorist' penny-pinching behaviour helped solve the first terrorist attack against New York's World Trade Center.
Around lunch time on Feb. 26, 1993, there was a powerful blast in an underground parking lot of the centre's North Tower, killing six people and injuring more than 1,000 others.
Later that afternoon, a man named Mohammed Salameh showed up at a New York Ryder outlet and said the van he had rented had been stolen. He asked for his $400 security deposit but was told he needed to return with a police report.
At the World Trade Center, investigators had recovered remains of a Ford Econoline van and determined from the damage that it was the vehicle that held the bomb. Two metal fragments, including a section of the frame rail, carried the van's Vehicle Identification Number. This help identify the Econoline as the van rented by Mr. Salameh.
On March 1, Mr. Salameh had purchased a plane ticket to Amsterdam but, still eager to reclaim his $400, he returned to the Ryder rental counter to get his deposit. FBI agents were waiting for him and catching him led to the arrest of four other suspects.
Some aspects of the bombing of the Alfred P. Murrah Federal Building in downtown Oklahoma City were similar to the World Trade Center attacks and there was public speculation that Muslim radicals were behind the April 19, 1995, attack, which killed 168 people, including 19 children.
As in 1993, a rented Ryder truck had been packed with homemade explosives. The next day, the FBI had recovered a rear axle with a Vehicle Identification Number. This led agents to a body shop whose employees were able to help police produce a composite sketch of the man who had rented the van. Hotel employees were shown the sketch and one connected it to a traveller, a former U.S. Army soldier named Timothy McVeigh.
Investigators then discovered that Mr. McVeigh was already in custody. Less than 90 minutes after the blast, he was driving away, 110 kilometres north of Oklahoma City, when he was pulled over by a state trooper for driving without a front licence plate. The officer also noticed a bulge under Mr. McVeigh's jacket and found a loaded Glock pistol.
Mr. McVeigh was detained on firearms charges and for motor-vehicle offences. He would have been promptly released on bail but the local court was overcrowded and his hearing was delayed by two days, by which time the FBI had named him as a suspect.
He was executed by lethal injection in June, 2001.
THE MADRID TRAIN ATTACKS
Some commuters who took the train in Madrid at rush hour on March 11, 2004, noticed bag-carrying men whose heads were covered by hats and scarves despite the warm weather. Minutes later, at 7:40 a.m., the bombs the men had left behind detonated on four different trains, killing 191 passengers and wounding 1,900 others.
The government initially blamed Basque separatists. Ten bombs had exploded and three others that failed to go off were destroyed on the spot as a safety measure, leaving little forensic evidence.
The big break came after police collected luggage left behind at one bombing scene, the El Pozo station outside the capital. Officers spotted in one bag a cellphone connected to two dangling wires. Bomb squad technicians were called and uncovered 10 kilos of dynamite attached to a detonator and the mobile phone. The bomb had failed because the timer was wrongly set to 7:40 p.m. rather than 7:40 a.m.
Investigators linked the subscriber identification module (SIM) on the phone to a Madrid shop run by Moroccan-born Jamal Zougam. By retracing other SIM cards from the same batch as the one in the unexploded bomb, police closed in on a flat in the Madrid suburb of Leganes but the men inside blew themselves up in April. Four suspects and one police officer died.
Survivors identified Mr. Zougam as one of the men who loaded bags onto the trains and he was one of three men who faced the toughest sentences and would serve a maximum of 40 years behind bars. Prosecutors failed to clinch a conviction against six other defendants, including the alleged mastermind, Rabei Osman Sayed Ahmed. He was however already serving a ten-year sentence on unrelated terrorism charges in Italy.