Warning that "Americans are still at risk on the ground," Gordon Duguid, a State Department spokesman, confirmed the deaths of two Americans in Mumbai.
"The families have been notified by the State Department," Duguid said, but he would not name the victims or comment further on them.
U.S. officials were checking with Indian authorities and hospitals to learn more about the extent of casualties.
A spokeswoman for a meditation group in Virginia said two Americans traveling with the organization in Mumbai were among 22 foreigners who died there, and a U.S. official who spoke on condition of anonymity verified that account. Bobbie Garvey, the spokeswoman for the Synchronicity Foundation, based in Faber, Va., identified the two slain members as Alan Scherr, 58, and his daughter, Naomi.
The meditation group said in a statement that four members of the 25-person group - two Americans and two Canadians - who were staying at the Oberoi Hotel were wounded by gunfire, and were believed in stable condition.
The State Department urged Americans not to travel to the stricken city, at least through the weekend.
A U.S. investigative team planned to leave for Mumbai on Friday, a State Department official said, speaking on condition of anonymity because the U.S. and Indian governments were working out final details of the cooperative effort. The official declined to identify which agency or agencies the team members came from.
Department spokesman Robert McInturff said Thursday he could not identify those injured, but The Associated Press learned the name of one victim: Andi Varagona of Nashville, Tenn. She called her mother from a hospital Thursday and said she had been shot in the arm and leg while eating dinner at the Taj Mahal Palace and Tower hotel.
She said another Tennessee woman traveling with her was also injured, according to the mother, Celeste Varagona, but the woman's identity was not immediately available.
McInturff said U.S. officials have activated a phone tree to contact American citizens who registered with the U.S. consulate in Mumbai.
A South Asia specialist said Friday that the terror "group itself is probably drawing from, in large numbers, Indian operatives, but it probably enjoys a fairly healthy support of Pakistan."
"The big picture is that there's probably going to be more of this, not less of this, to come," Christine Fair of the RAND Corp.
said Friday. "I don't think this is the most lethal attack that terrorists have perpetrated, but it is certainly the most expansive, in its scope and its scale and its perplexity."
Since 2005, India has been cautious when talking about the terrorist attacks it has suffered, referring to the involvement of "neighbors" or "external forces," Fair said, in an attempt to settle a coveted U.S.-Indian civilian nuclear cooperation deal.
"They wanted to appear to be responsible," Fair said. "Now this deal has been secured, so some of the restraint, in terms of the rhetoric, in terms of the response - the pressures for that restraint may have been eliminated."
"It would be premature in view of the unfolding tragedy in Mumbai and the corresponding investigation to reach any hard-and-fast conclusions on who may be responsible for the attacks, but some of what we're seeing is reminiscent of past terrorist operations undertaken by groups such as Lashkar-e-Taiba and Jaish-e-Mohammed," a U.S. counterterrorism official said on condition of anonymity. The two groups mentioned by the official are Pakistani militants linked to al-Qaida who have fought Indian troops in Kashmir.
President-elect Barack Obama spoke Thursday by telephone with Secretary of State Condoleezza Rice and received several intelligence briefings.
President Bush expressed condolences to Indian Prime Minister Manmohan Singh in a phone call from the Camp David, Md., presidential retreat, where he remained Friday.
"The president offered support and assistance to the government of India as it works to restore order, provide safety to its people and comfort to the victims and their families and investigate these despicable acts," press secretary Dana Perino said in a statement.
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.