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.

“The train stopped for a long time and there was an announcement about signal failure, but I didn’t realize anything strange was going on until later,” Chase-Levenson wrote in an e-mail.

“Fortunately, I had decided to walk to a different tube stop rather than changing at King’s Cross which I often do,” he said.

Ruthie B. Birger ’06, who lives two tube stops from King’s Cross, wrote in an e-mail that “when I got to the [Finsbury Park] station there was an announcement that it was [closed] and that we should take the buses....At the time I thought it was only my tube station that was closed but as the bus drove through north and central London I saw hundreds of people waiting at the stops so I realized something else was going on.”

At 9:17 a.m., there was a third explosion involving two or three trains around Edgware Road station. Seven people were killed in that attack, police said.

The fourth and final attack, on a double-decker bus, came at 9:47 a.m. near Russell Square, killing at least two, according to the police.

The attacks came only a day after the start of a G-8 summit in Scotland attended by several heads of state, including Blair and President George W. Bush. Blair rushed back from the summit to London for emergency meetings, appearing in a televised address later that afternoon calling the day “a very sad day for the British people.”

BOSTON RESPONDS

Shortly following the attacks in London, Massachusetts Governor Mitt Romney met with state transportation officials, ordering the Massachusetts Bay Transit Authority (MBTA) to increase security system-wide, marking the first such security upgrade in the state since last summer’s Democratic National Convention.

MBTA police inspected tracks, out-of-service trains, and buses for bombs; recordings played over speakers at Park Street directed commuters to report suspicious behavior or packages.

Later in the day, the Department of Homeland Security raised the threat level for the nation’s transit systems to code orange, which is the second highest level of alert. Although officials had received no specific threats against U.S. cities, Homeland Security Secretary Michael Chertoff said that he remained “concerned about the possibility of a copycat attack.”

HARVARD TAKES ACTION

Thousands of miles from London, officials at Harvard, watching television news reports of the London blasts for the first time, rushed to contact every Harvard student they believed to be in the London area.

The University has no central database listing students’ summer locations, and not all students in London are necessarily registered through Harvard. Students can work or attend classes abroad under the auspices of many University offices, including the Office of International Programs (OIP), the Office of Career Services (OCS), the Minda de Gunzburg Center for European Studies (CES), and the Summer School. Officials from several of these programs worked together Thursday to contact all of the students believed to be in the London area, as well as those believed to have plans to travel to London later in the summer.

Around 3 p.m. London time, CES sent an e-mail to several of its students in London, asking them to “please take a few seconds to let us know how you are.”

Trisha Craig, the executive director of CES, said that “happily, we have heard from almost everyone [in London], at least the people who will be able to get to e-mail.”

Craig said her office was tracking 12 undergraduates and seven graduate students, and had contacted the students’ places of work or study in addition to sending them personal e-mails.

Leslie M. Hill, an assistant director at the OIP, said she sent two e-mails to Harvard students in London, at around 4 and 10:30 p.m. London time. In the second e-mail, Hill asked students to “send me a reply to let me know that you are safe and well.”

Throughout the day yesterday, The Crimson received dozens of e-mails from College students who were in London during the attacks. Many lauded University administrators for their concern and responsiveness, and one student noted that Harvard’s response to the attacks had been “great” in comparison with that of the London School of Economics, where he is studying this summer.

But four students, all working or living in London for the summer, contacted The Crimson after the OIP sent out its first e-mail, saying they had not yet received any communication from Harvard.

Officials at the Secondary School Program (SSP) confronted problems of a different kind yesterday—several British students, with families potentially in London, are currently taking classes on the Harvard campus through the SSP.

SSP Director William Holinger said that he had discussed the London attacks at the daily meeting of SSP deans, and Summer School proctors had been directed to talk immediately with British SSP students about their family situations. If students were unable to contact their families due to the stress the emergencies placed on the London telephone network, Holinger said that the SSP was ensuring that students had access to international phone lines.

Another component of the Summer School—study abroad—had scheduled a month-long session of classes at Oxford University beginning July 24.

In an e-mail sent at around 9 p.m., London time to students registered for the Oxford program, Robert Neugeboren, a Summer School assistant dean, wrote that “we are not aware of any [Summer School] students having been in London at the time of the incident. We are following the situation closely and will keep you up to date on any foreseeable impact” on the upcoming program.

Neugeboren said that there are currently no plans to cancel the program. Several Harvard students who are currently living in London said that they had made no changes to their summer plans because of the attacks, and intended to stay in London for the completion of their programs.

But for others, the attacks have changed plans completely.

Erik C. Belz ’08, who had been taking a class at the London School of Economics, wrote that “my parents do not feel comfortable with me continuing to study here this summer and so I will be returning to Boston tomorrow.”

—Material from the Associated Press was used in the reporting of this story.

—Staff writer Natalie I. Sherman contributed to the reporting of this story.

—Staff writer Brendan R. Linn can be reached at blinn@fas.harvard.edu.