Injured tube passengers are escorted away from the Edgware Road Tube Station in London following an explosion yesterday morning. Three blasts went off on the London subway and one went off on a double-decker bus.

As Londoners reeled in the wake of four rush-hour explosions that killed at least 37 people and wounded more than 700 yesterday morning, University officials scrambled to locate dozens of Harvard students working and taking classes in the city for the summer.

It was not immediately clear which organization was responsible for the attacks, which were the deadliest the city has seen since the Nazi blitz of 1940-1941, although a group calling itself “The Secret Organization of al Qaeda in Europe” posted a message on an extremist website taking credit for the attacks, calling the bombings “a response to the massacres carried out by Britain in Iraq and Afghanistan.”

A senior U.S. counterterrorism official said that online claim was “potentially very credible,” and several British authorities, including Prime Minister Tony Blair, suggested Islamic extremists were to blame for the carnage.

The four bombings—three targeting Underground trains and one a double-decker bus—struck within an hour of each other yesterday morning. British authorities said they had no warning of the attacks. The New York Times reported that the charges were detonated by timers rather than suicide bombers.

University administrators spent much of yesterday attempting to contact every Harvard student they believed was in London at the time of the attacks. Faculty of Arts and Sciences spokesman Robert Mitchell estimated that the number of undergraduates in the London area to be around 70, and the number of graduate students to be between 60 and 70.

Mitchell and other University officials were unable to say how many students Harvard has confirmed are safe.

There are several different programs under which students are studying or working in London. Although most of the programs kept their own records of students studying abroad, University officials yesterday were working to consolidate those lists into a comprehensive database.

“A VERY SAD DAY”

The first blast came at 8:51 a.m. London time aboard an Underground train between Moorgate and Liverpool Street stations, near the city’s financial district. Seven people died in that attack, according to London police.

Vijay Jethwa ’06, who is living at home in London this summer, was jogging along the Thames yesterday morning at around the time of the first explosion.

“About half an hour into my workout, I saw lots of police cars and vans along the Embankment,” a tube (London slang for Underground) station five stops from the stricken Liverpool Street location, Jethwa wrote in an e-mail.

“On the jog back home, I saw many commuters stranded outside the tube station. This [was] the point when I started to get worried heading promptly back home,” he said.

Then, five minutes later, at 8:56 a.m., a second Underground train, between King’s Cross and Russell Square stations, was bombed. Police said 21 people died in that attack.

Alexander N. Chase-Levenson ’08, who is working at Regent’s College for the summer, was riding on the Underground around the time of the King’s Cross explosion.