LONDON (AP)- Commuters in London reluctantly descended into the Underground on Friday morning, attempting to return to routine in the aftermath of four rush-hour blasts that killed at least 50 people. Police said the attacks had the signatures of the al-Qaeda terror network.

There was nothing to suggest suicide bombers carried out Thursday”s attacks, but the investigation was still in its early stages, said Sir Ian Blair, commissioner of the Metropolitan Police. &#34We have absolutely nothing to suggest this was a suicide bombing attack although nothing at this stage to rule that out,&#34 Blair told a news conference.

Each bomb was packed with less than 10 pounds (4.5 kilograms) of high explosives, little enough to fit in a backpack, police said. Police denied that they had found any unexploded devices. On Thursday, a senior U.S. counterterrorism official who spoke on condition of anonymity because of the sensitivity of the information had said British authorities identified suspicious packages and detonated them in controlled explosions.

At least 50 people were killed in the attacks, police said Friday. One day earlier, the toll stood at 37, and police said more bodies had yet to be recovered. It was unclear whether recoveries would push the toll higher.

More than 700 were injured in Thursday”s attacks on London, the worst attack in the city since World War II.

Police said investigation and recovery were hampered at one bomb site, a subway carriage near the Russell Square station where 21 people were killed, because of fears the tunnel was unsafe. Engineers were examining structural damage.

Security officials locked patrons inside the British Museum briefly after guards spotted two suitcases found chained together inside the museum. Doors were unlocked after they located the bag”s owners, Korean tourists who were visiting the renowned museum. Security was tight at the museum with guards checking visitors” bags. Few visitors were inside at the time. Normally, the museum is packed on Fridays.

Investigators were looking for evidence in the debris and in the video footage from some 1,800 cameras in London”s train stations. &#34Our total effort today is focused on identifying the perpetrators and bringing them to justice,&#34 Home Secretary Charles Clarke said Friday. &#34That is, of course, the No. 1 preoccupation that the police and security services have at this moment.&#34

Thursday”s blasts went off in an 18-minute period at three subway stations, starting at 8:51 a.m. An explosion ripped the roof off a double-decker bus less than an hour later, attacks that came as world leaders were opening the G-8 summit in Scotland. &#34I didn”t hear anything, just a flash of light, people screaming, no thoughts of what it was, I just had to get out of the train,&#34 said subway passenger Chris Randall, 28, who was hospitalized with cuts and burns on his face, legs and hands.

Prime Minister Tony Blair, who just the day before had been basking in glory of Britain”s successful Olympics bid, condemned the attacks and blamed Islamic extremists.

Foreign Minister Jack Straw said the attacks bore the hallmark of Osama bin Laden”s al-Qaeda, the group responsible for the well orchestrated Sept. 11, 2001, attacks.

Queen Elizabeth II and her son Prince Charles visited victims at the crowded St. Mary”s Hospital. &#34It”s been one of the things that many of us have dreaded for a long time,&#34 Charles said, speaking to patients injured in the blasts. &#34What I can never get over is the resilience of the British people who have set us all a fantastic example of how to recover.&#34

His wife Camilla, Dutchess of Cornwall, said people”s efforts &#34makes me very proud to be British.&#34

Stocks opened higher in Europe on Friday, with insurance and travel-related stocks regaining some of the ground they lost on Thursday.

The attacks shut down the mass transit system on Thursday.

On Friday, 10 of London”s 12 subway lines reopened Friday, though service on three was restricted. Bus service was running through central London, except for diversions around blast sites. But some commuters chose to take a taxi. &#34Normally, I take the bus, but today, I took a taxi. I was a bit afraid,&#34 said Aldona Mosjko, a 21-year-old bagel shop manager from Poland.

Others said they had little choice but to return to the Underground. &#34I was scared, but what can you do?&#34 said Raj Varatharaj, 32, emerging from a subway station. &#34This is the quickest way for me to get to work. You just have to carry on.&#34

Some commuters commented on what appeared to be a light police presence at some Underground stations. &#34Everyone is very quiet, everybody is a bit anxious,&#34 said Anil Patel, 40, a banker. &#34An obvious police presence would have settled your nerves.&#34

Investigators were examining a Web site claim in which an organization calling itself the &#34Secret Group of al-Qaeda”s Jihad in Europe&#34 said it was behind the attacks. That claim could not be immediately verified but was being taken seriously, Clarke said.

The group called the bombings punishment for Britain”s involvement in the war in Iraq and invasion of Afghanistan. It said Italy and Denmark also were being warned for their support of the U.S-led coalitions in both countries.

A senior U.S. counterterrorism official said the posting was considered a &#34potentially very credible&#34 claim, in part because it appeared soon after the attacks.

Charles Shoebridge, a security analyst and former counterterrorism intelligence officer, said police would no doubt want to act quickly in trying to hunt down the attackers. &#34There is real passion now in the police to make arrests quickly before further attacks can be carried out,&#34 he said.

Shoebridge said detectives will have to watch thousands of hours of video, slowly and carefully. Investigators will try to find on tape the point at which bombs were placed, then trace back the movements of the bomber, a task he said which could involve hundreds of cameras.

He said investigators will also check records of cell phone calls made in the bombed areas just before the explosions, a job that might be difficult if investigators cannot determine where bombers boarded the trains.

Based on evidence recovered from the rubble, investigators believe some of the bombs were on timers, a U.S law enforcement official said. The official would not further describe the evidence. Investigators doubt that cell phones that were used in the Madrid train attacks a year ago, were used to detonate the bombs in the Underground because the phones often don”t work in the system”s tunnels, the official said. The official spoke on condition of anonymity because the investigation is ongoing.