Reeling from a historic earthquake and tsunami, Japanese authorities struggled Saturday to deal with a humanitarian crisis, a still-untamed nuclear power plant and emerging doubts about the government’s credibility and competency.

Across the Pacific, trace amounts of the radioactive isotope xenon-133 lit up a sensitive detector in Sacramento, and scientists said it was likely from the crippled Fukushima Daiichi nuclear plant, but the amount was not nearly enough to affect human health. U.S. officials said the dose rate was about one-millionth of what a person “normally receives from rocks, bricks, the sun and other natural background sources.”

Japan, however, continues to suffer from the lethal combination of natural and technological disasters. The death toll from the March 11 quake and tsunami reached 7,197, with 10,905 missing, according to the National Police Agency.

Prime Minister Naoto Kan sought to assure his countrymen that Japan will rebuild. But his words came amid doubt that the nation’s leaders have a firm grip on the nuclear crisis. The government and the Tokyo Electric Power Co. have issued a thin and fitful stream of information about the radiation-spewing plant.

In recent days, officials in Tokyo and Washington have sent different signals about the level of hazard posed by the damaged nuclear reactors. U.S. officials have advised Americans to evacuate from a much broader region of Japan. Japanese authorities implicitly acknowledged Friday that they had underestimated the severity of the nuclear crisis, as they re-categorized the emergency as a level-5 event, up from level 4, on the International Nuclear Event Scale. That is still shy of a level 7 catastrophe, which would be akin to the 1986 Chernobyl event in Ukraine.

Japanese emergency workers ramped up efforts to stabilize the rapidly deteriorating Fukushima Daiichi nuclear plant Saturday, racing to reestablish electrical power to portions of the facility and establishing an automated water canon to drench two reactors for up to seven consecutive hours.

Cabinet Secretary Yukio Edano said the renewed attempts to regain control had shown signs of success at the No. 3 nuclear reactor, the highest priority for officials aiming to cool spent fuel rods that have begun spewing radioactive material in the atmosphere. But he acknowledged that the gains could be temporary.

“As of now we cannot say anything definite, but we think we have succeeded in putting a certain level of water in Unit 3 and we think that it is in a certain stable situation,” Edano said. “We have been able to prevent the situation from worsening ... but I believe we are reaching a big turning point.”

Meanwhile, Edano said, milk and spinach found within nearly 20 miles of the damaged plant in Ibaraki and Fukushima prefectures were found to have higher than normal levels of radiation contamination after initial tests by government scientists.

Although he stressed that the amounts of radiation posed no health risk for humans, Edano said further tests would be conducted and that the government would ban any contaminated food products from the marketplace. The ingestion of milk by children near Chernobyl, after an accident at the Soviet nuclear facility in 1986, was blamed as the cause of thousands of cancer cases.

At the Daiichi plant, emergency crews set up a fire truck with a 70-foot-tall water cannon that sprayed 3 tons of water per minute on the No. 3 nuclear reactor. That is the only one of the six reactors to use plutonium, which is considered more dangerous than uranium.

The spraying, done without the need for humans to be in the dangerous zone of elevated radiation, began at about 2 p.m. Saturday local time and was to continue for about seven hours, officials said, with the water split between reactors No. 3 and No. 4. Both were severely damaged by the 9.0-magnitude earthquake March 11 and subsequent tsunami.

Meantime, officials were working to reestablish electrical power to reactor No. 2, an effort that would allow cooling efforts to resume in that facility, which has suffered less damage.

After so many days of dire bulletins from the nuclear plant, Saturday arrived in Japan with what passed for good news: There were no new explosions, no new fires. Radiation levels within the Fukushima Daiichi plant remain dangerously high, limiting the amount of time workers can be exposed before they run the risk of radiation poisoning. Still, intrepid workers braved the invisible atomic storm in relay teams and managed to attach a new electrical line to the blacked-out facility.

That doesn’t mean power is restored. TEPCO hoped to restore electricity to one of the nuclear reactor units by the end of the day Saturday, but many steps may be necessary before the full facility has power. Even once it does, the company does not know whether the cooling system that circulates water can be made operable.

U.S. officials want their own eyes on the situation. The Nuclear Regulatory Commission has dispatched 11 of its own technical experts to Japan in order to “shorten the information chain,” said NRC spokesman Scott Burnell.

Relying on the data the NRC’s experts collected, Commission Chair Gregory Jaczko stunned lawmakers on Capitol Hill this week when he said a pool at unit 4 of the Fukushima Daiichi plant no longer contained water to cool spent fuel, making it more likely that it would emit radiation. His comments contradicted those of Japanese and TEPCO officials, who continue to maintain that there is water left in the pool.

Other nuclear experts expressed frustration with the amount of information the Japanese government and TEPCO officials have released. “I think the Japanese government is sometimes not as forthright as they should be,” said Mark Pierson, a nuclear engineering professor at Virginia Tech. Pierson speculated that U.S. officials have access to information via military satellites that enable them to collect information the Japanese don’t have.

“The international community’s view is that they want more volume of accurate information more quickly,’’ Yukiya Amano, chief of the International Atomic Energy Agency (IAEA), said after meeting Kan and other Cabinet ministers.

Asked about public doubts that the government is telling the whole truth about the nuclear crisis, Kan said later: “We have been disclosing all facts that I and the cabinet secretary are able to get hold of regarding the power plant accident.”

Japanese officials acknowledged that they had failed to anticipate so great an earthquake and so destructive a tsunami. They also faulted themselves for a slow and disorganized response to the natural disaster.

“The unprecedented scale of the earthquake and tsunami that struck Japan, frankly speaking, were among many things that happened that had not been anticipated under our disaster management contingency plans,” Chief Cabinet Secretary Edano said. “In hindsight, we could have moved a little quicker in assessing the situation and coordinating all that information and provided it faster.”

The fight to cool the overheated reactors and spent fuel rods remained a primitive one, with firetrucks shooting streams of water from a distance. The trucks, including one lent to Japan by the U.S. military, took aim at the unit 3 reactor building, which is emitting the most radiation and is the only unit that contains the deadly element plutonium. Each truck, equipped with a high-powered hose, stayed close to the plant for only a few minutes to limit radiation exposure.

Nuclear experts point out the Fukushima plant — damaged by the earthquake, the tsunami, and then a series of still-mysterious explosions — could have structural damage that makes the power reconnection difficult. The lack of electricity has limited the options for the electric company. “They can only take symptomatic measures,” said Hiroshi Kimura, a nuclear expert at the University of Tokyo. “For now, we have to depend on the spent fuel to cool itself from vapor evaporation by showering water into the pool.”

Water plays two key roles here. It cools the fuel rods, which, despite no longer undergoing nuclear fission, still throw off heat due to the natural decay of radioactive elements. It also provides shielding from radiation that has made the site an extraordinarily dangerous place to operate.

The nuclear industry has a limit for radiation doses during normal operations and a higher limit that can be imposed in an emergency, said Kim Kearfott, a professor of nuclear engineering at the University of Michigan. She said 46 workers died from radiation poisoning within weeks after the Chernobyl accident. So far, she said, Japanese authorities appear to be carefully managing the exposure of workers to radiation.

“If they have good radiation safety practices, they could avoid the deaths of workers from acute radiation syndrome,” Kearfott said.

With many Americans, especially those living on the West Coast, rushing to stock up on potassium iodide pills that protect, the U.S. Environmental Protection Agency on Friday announced that it has created a new Web site to keep the public updated on data collected by its RadNet system, which has been bolstered to better monitor air samples, especially from Western states, for any signs of radiation arriving from Japan.

In Japan, radiation-related fears have prompted foreigners to flee, and broadcaster NHK reported that about 10,000 Japanese people have already abandoned their homes to get farther away from Fukushima.

For millions more — especially those who do not live in the areas hit hardest by the earthquake and tsunami — watching grainy television footage of dangerous efforts at Fukushima Daiichi has become a daily routine.

At 2:46 p.m. Friday, the country observed a moment of silence to mark the tragedy of seven days earlier.

Almost 11,000 buildings have partially collapsed, and roughly 500,000 people are displaced. Tokyo’s busiest streets are quiet, and its supermarkets are out of milk. Thousands of people in the hardest-hit areas are still without heat and basic supplies, and concerns about Fukushima’s radiation have left at least some survivors in a zone where aid workers are fearful of going.

Some disaster modeling companies have estimated the total insured loss at between $15 billion and $35 billion, with reconstruction costs in the range of $180 billion. That is about 3 percent of the country’s economic output, making this a costlier disaster than the 1995 earthquake in Kobe.

The crisis at the six-reactor nuclear plant most damaged by the disasters has added a terrifying volatility to Japan’s grief, Prime Minister Kan said in a nationally televised address Friday. The Japanese people “are being tested . . . We must not be discouraged by this earthquake and tsunami,” Kan said. “Let us confront this crisis together, with determination that we will once again rebuild Japan.”

He vowed: “We will rebuild Japan from scratch.”

Achenbach reported from Washington. Correspondent David Nakamura and special correspondent Akiko Yamamoto and Kyoko Tanaka in Tokyo and staff writers Annys Shin and Rob Stein in Washington contributed to this report.