President Obama made an unscheduled stop at the Japanese Embassy to sign a condolence book, writing, “My heart goes out to the people of Japan during this enormous tragedy.” He added, “Because of the strength and wisdom of its people, we know that Japan will recover, and indeed will emerge stronger than ever.”

Later, he appeared in the Rose Garden at the White House to offer continued American support for the earthquake and tsunami victims, and technical help at the nuclear site.

But before the recovery can begin, the nuclear plant must be brought under control. On Friday, steam that was likely laced with radioactive particles was again rising over the plant, this time billowing from reactor No. 2, which suffered an explosion Tuesday. But Japanese authorities said they did not yet know the cause of the latest release.

American officials, meanwhile, remained fixated on the temperature readings inside that reactor and two others that had been operating until the earthquake shut them down, as well as at the plant’s spent fuel pools, looking for any signs that their high levels of heat were going down. If the fuel rods are uncovered and exposed to air, they heat up and can burst into flames, spewing radioactive elements.

So far the officials saw no signs of dropping temperatures. And the Web site of the International Atomic Energy Agency, the United Nations nuclear watchdog, made it clear that there were no readings at all from some critical areas. Part of the American effort, by satellites and aircraft, is to identify the hot spots, something the Japanese have not been able to do in some cases.

Critical to that effort are the “pods” flown into Japan by the Air Force over the past day. Made for quick assessments of radiation emergencies, the Aerial Measuring System is an instrument system that fits on a helicopter or fixed-wing aircraft to sample air and survey the land below.

Daniel B. Poneman, the deputy secretary of energy, said at a White House briefing on Thursday that preliminary results of the initial flights “are consistent with the recommendations that came down from the chairman of the Nuclear Regulatory Commission,” which led to the 50-mile evacuation guideline given to American expatriates. Although the worst contamination is closer to the plant, the recommendation takes into account the possibility of shifting winds or greater emissions.