LONDON – A group of would-be bombers targeted three Underground trains and a red double-decker bus Thursday in a coordinated attack seemingly designed to mirror bombings two weeks ago that killed 56 people. The attacks caused no serious casualties and little damage.

Witnesses said bombs concealed in backpacks made loud popping sounds and emitted an acrid smell, but failed to detonate. In the panic and confusion that ensued, all four attackers fled.

The attacks came on the two-week anniversary of the July 7 bombings and clearly seemed intended to reproduce them. Besides involving backpack bombs, they were nearly simultaneous, and the targets were once again three subways and a double-decker bus distributed in a rough cross centered on the heart of London. That configuration could evoke the “flaming cross” described in a claim of responsibility on an extremist Islamic Web site after the first attacks.

It was unclear why the devices failed to explode and whether the attacks were part of a conspiracy to stage a terror campaign in London or a copycat attack.

Security officials speculated that the explosives used Thursday might have been too old, or that the bomb-maker made a mistake in putting the explosives together.

Authorities said the failed attempts at the Shepherd’s Bush station in West London, Warren Street in central London and the Oval, south of the Thames, could help police because the unexploded bombs and bags that concealed them provided valuable physical evidence. Closed-circuit television images could also lead to the individuals or group that has put London’s commuters in its sights.

“I feel very positive about some of these developments,” Metropolitan Police Chief Ian Blair said after the midday bombings that closed down several major Underground lines and left thousands of people stranded in central London. “Clearly, the intention must have been to kill. … The important thing is that the intention has not been fulfilled.

“From what I understand, some of the devices remained unexploded, if I can describe it that way,” Blair said. “The explosives officers and the forensics and everybody else is going to take their time to examine all of this – which ones are detonated, which parts have gone off of the explosives – we are just going to have to be patient.”

First reports of the attempts came about 12:30 p.m.

The affected Tube lines were quickly closed down, the affected stations were evacuated, and police cordoned off several city blocks.

Passengers on the affected trains and bus described moments of sheer terror.

“I was sitting on the Tube not paying much attention to anything and I heard a pop, like a really big balloon had burst, then I saw a little smoke,” passenger Kate Reid told the BBC. “I saw a bag on the floor next to a young man who looked really scared. We pulled into the Oval station and he just sprinted away as soon as the doors opened.”

Outside Warren Street station, passenger Sufiane Mohellavi, a Frenchman, said he was on a Victoria Line train when he suddenly smelled burning, like an electrical fire. He said he did not see smoke or hear an explosion, but there was a “truly terrible odor.”

“When I was in the train it was agonizing,” he said. “I wanted to jump, get out of the carriage, break a window and jump out.”

As the train pulled into the station, passengers raced for the doors. “All I could think about is when am I going to get out,” Mohellavi said.

A small explosion aboard a No. 26 bus in Hackney, in east London, blew out a window on the upper deck, but no passengers were injured. Police kept spectators away as bomb experts focused their attention on a suspicious piece of baggage.