Intelligence officials looked urgently for clues about the identity of the attackers, a crucial unknown as Indian officials charged, without giving details, that "elements in Pakistan" were involved. A tentative rapprochement between the two nuclear-armed rivals could hang in the balance, and a U.S. counterintelligence official cautioned against rushing to judgment on the origins of the militants.
President George W. Bush pledged cooperation with Indian authorities and mourned the deaths of more than 150 people at the hands of gunmen who attacked targets across India's financial capital starting Wednesday night.
"My administration has been working with the Indian government and the international community as Indian authorities work to ensure the safety of those still under threat," he said in a statement from the Camp David presidential retreat in Maryland. "We will continue to cooperate against these extremists who offer nothing but violence and hopelessness. "
A U.S. counterterrorism official said it was premature to reach conclusions on who may be responsible for the attacks. The official, who spoke on condition of anonymity because of the sensitivity of the investigation, said some "signatures of the attack" were consistent with the work of militants who have fought against India in the disputed Kashmir region.
Officials were working out the final details with Indian diplomats Friday for the departure of an FBI team, said U.S. authorities, who also spoke on condition of anonymity because of the delicate nature of the operation. A second group of investigators was on alert to join the first team if necessary.
The investigators aim to learn more about the militants who carried out the lethal strikes on luxury hotels, a train station and an Orthodox Jewish center where a rabbi and his wife, who had moved to Mumbai from New York, were among five hostages slain. An American and his teenage daughter traveling with a Virginia-based spiritual group were also among those killed during the coordinated attacks.
"Americans are still at risk on the ground" in Mumbai on Friday, the State Department said Friday, warning citizens not to travel to the stricken city at least through the weekend.
U.S. officials were checking with Indian authorities and hospitals to learn more about the extent of casualties.
In New York, the Chabad-Lubavitch movement confirmed Friday that Rabbi Gavriel Noach Holtzberg and his wife, Rivkah, were among those killed in the terrorist assault on the ultraconservative group's Mumbai headquarters.
A spokeswoman for a meditation group in Virginia said two Americans traveling with the organization in Mumbai also were killed. Bobbie Garvey, speaking for the Synchronicity Foundation, based in Faber, Va., identified the two slain members as Alan Scherr, 58, and his 13-year-old daughter, Naomi.
State Department spokesman Robert McInturff said U.S. officials have activated a phone tree to contact American citizens who registered with the U.S. consulate in Mumbai.
Pakistan's ambassador to the U.S., Husain Haqqani, said in a statement that his country is "confronting the menace of terrorism with great vigor." Haqqani insisted "it is unfair to blame Pakistan or Pakistanis for these acts of terrorism even before an investigation is undertaken."
India and Pakistan have fought two of their three wars over Kashmir. U.S. officials are concerned about a flare-up in animosity similar to one that occurred after Pakistani militants attacked the Indian parliament in December 2001, officials said.
Underscoring those fears, Secretary of State Condoleezza Rice has called the foreign minister of India twice, along with Pakistani President Asif Ali Zardari, since the crisis began.
"There were very worrying tensions in the region," said Gordon Duguid, a State Department spokesman. "She was calling the president of Pakistan to get his read on how those tensions might be affected."
Democratic Sen. Russ Feingold of Wisconsin said that "as we continue to learn the details about the attacks and those responsible for them, we must not allow them to undermine the progress that has been made to foster better relations between India and its neighbor Pakistan, two critical partners in our global fight against terrorism."
President-elect Barack Obama has spoken by telephone with Rice about the attacks and received several intelligence briefings, State Department officials said. They said Rice spoke again Friday with Indian Foreign Minister Pranab Mukherjee.
"These terrorists who targeted innocent civilians will not defeat India's great democracy, nor shake the will of a global coalition to defeat them," Obama said in a statement. "The United States must stand with India and all nations and people who are committed to destroying terrorist networks, and defeating their hate-filled ideology."
The State Department set up a call center for Americans concerned about family members who may be in Mumbai. The number is 1-888-407-4747.
Associated Press writers Matthew Lee, Pamela Hess and Sharon Theimer in Washington, Tom Breen in Richmond, Va., and Juanita Cousins in Nashville, Tenn., contributed to this story.