LONDON -- "Clearly, the intention must have been to kill" -- the words of Police Commissioner Ian Blair on Thursday's London explosions.

Small explosions struck the London Underground and a bus at midday in a chilling but bloodless replay of the suicide bombings that killed 56 people two weeks ago.

No one was injured in the coordinated lunch-hour blasts, which shocked and disrupted the capital; the explosive devices were either faulty or too small to cause bloodshed.

Two men were arrested and questioned in the four attacks, but one of the two was later released without charge.

Authorities said it's too early to determine whether the attacks were carried out by the same organization behind the deadly blasts two weeks ago -- or whether they were linked to al-Qaida.

But Blair said earlier that forensic evidence collected from the crime scenes could provide a "significant break" in solving the case.

British authorities reportedly told their U.S. counterparts that backpacks and explosives used Thursday were identical to those in the July 7 attacks.

Also reported was the "speculation" that the devices were so similar, they may even have been part of the same batch.

Blair said some of the bombs apparently didn't go off. He said Thursday's explosions were "pretty close to simultaneous," but that the bombs appeared to be smaller than the ones that exploded in the city exactly two weeks ago.

Blair also said initial checks have yielded no trace of chemical agents in two of the subway stations.

The U.S. Embassy in central London has been closed to visitors because of the new assault on the transit system, but a spokeswoman said embassy staff members were still working.

British television showed footage of the double-decker bus parked by the side of the road. In the footage, nobody was on board the bus or nearby -- and the streets appear to have been cordoned off.

The company that operates the bus said the driver heard a bang and went upstairs -- where he found the windows blown out. The company said the bus was otherwise intact, and that it had no reports of injuries.

Police said they're not treating it as a "major incident" on the level of the bus and subway attacks that killed more than 50 people two weeks ago. But the Warren Street, Shepherd's Bush and Oval stations have been evacuated.

Prime Minister Appeals For Calm

British Prime Minister Tony Blair appealed for calm Thursday in the aftermath of the explosions. He said he's going to resume his regular schedule because it's important to live as "normal" a life as possible.

He said "we can't minimize incidents such as this" -- but added that those responsible for the blasts are trying to scare people and make them worried.

However, the explosions sent a new ripple of fear throughout Britain.

"This is something London has to get used to," a 19-year-old man said.

And a 35-year-old woman said, "I am still shaking."

One man said his train was only 15 seconds from the station when he began smelling smoke. He said people began coming into his carriage. The witness said another passenger told him a backpack had exploded. He said it was a minor blast -- but enough to rip open the bag.

Another witness said he was on the way to a job interview when he began smelling smoke while traveling the Underground. The man said he was evacuated at the Warren Street Station. He said he didn't see smoke -- but smelled something similar to an electrical fire.

He said people began to scream and pulled the train's alarm.

Police cordoned off streets, and officers with sniffer dogs are checking the areas.

One woman who was evacuated from a subway station said she was on one of the trains -- and smelled smoke. She said, "It was like something was burning."

The woman said, "Everyone was panicked and people were screaming. We had to pull the alarm."

There's also a report that police are looking for someone at a London hospital in connection with the incidents.

An armed police unit has entered University College hospital. A British news agency said they arrived shortly after an injured person was brought in.

And Sky TV said police are looking for a man who was seen with wires protruding from his shirt.

Although nobody was killed in Thursday's London explosions, some residents said they're plenty jittery.

One man said the blasts were "a replay" of two weeks ago, when 56 people were killed -- including four suicide bombers.

"Everyone I know is buying bicycles now," he said.

Another man said he won't be taking the subway or bus to work Friday.

Traffic was backed up in the city in the aftermath of the blasts, with some underground train stations closed and offices evacuated.

Cell phone companies reported a surge in traffic, though not as big as two weeks ago. They urged customers to keep calls brief or use text messages instead.

Bush Briefed On London Explosions

Meanwhile, a White House spokesman said President George W. Bush has been briefed on the latest incidents in London's subway system.

Scott McClellan said officials are "monitoring the situation closely." He said the president was briefed by White House Chief of Staff Andy Card and national security adviser Stephen Hadley at the end of his regular intelligence briefing.

The Department of Homeland Security said Secretary Michael Chertoff is closely monitoring events in London, but a spokeswoman for the federal agency said Chertoff has no immediate plans to raise the U.S. terror alert.

U.S. mass transit systems have remained on Code Orange -- or high alert -- since the London bombings two weeks ago, but the rest of the country is at yellow, signifying an elevated risk.

In a speech that focused on trade, the president didn't specifically mention the latest incidents in London, but Bush said terror attacks won't shake the will of the free world. He said terrorists just don't understand that "when it comes to the defense of universal freedoms, this country won't be frightened."

He called terrorists "cold-blooded killers who embrace an ideology of hatred." He said their attacks are designed to "create vacuums" into which their ideology can move.

Man Escorted Away From Downing Street

Police in London escorted a man away from Downing Street, which is where the British prime minister lives and works.

A police officer near the end of the street drew a firearm and aimed it at a target beyond the range of television cameras.

Another officer then led away a man dressed in a black shirt and pants.

Pakistani Suspect Reportedly Had Links To London Bombers

Police in Pakistan have made what they call an important arrest in the search for the mastermind behind the London bombing attacks two weeks ago Thursday.

Pakistan, cooperating with British investigators, used a list of telephone numbers to determine who might have had contact with the suspected suicide bombers.

London newspapers identify the suspect arrested in Pakistan as Haroon Rashid Aswat. He reportedly visited the British hometowns of all four bombers and selected the targets.

Prime Minister Blair is holding an intelligence and security review Thursday as the government considers asking for new powers to prevent more attacks. It's already working on new anti-terror measures, including making it a crime to visit a terrorist training camp or glorify an act of violence.