Tokyo (CNN) -- Highly radioactive water continued leaking Sunday directly from the Fukushima Daiichi nuclear power facility into the Pacific Ocean, following a failed attempt to plug a cracked concrete shaft, an official with the plant's owner said.

Authorities said Saturday that they had discovered water gushing out of a two-meter-deep, concrete-lined basin and into the sea via a roughly 20-centimeter (8-inch) crack. The shaft sits behind the No. 2 reactor's turbine building at the facility, which has been in constant crisis since the failure of primary and back-up systems to cool nuclear fuel and numerous explosions in the wake of last month's epic earthquake and tsunami.

Later that day, power plant workers tried -- unsuccessfully -- to fix the leak by poring in fresh concrete, Tokyo Electric Power Company officials said late Saturday.

Now, they are on to "Plan B." An expert with the utility company planned to visit the site Sunday morning, after which it would be decided what type of polymer (a synthetic compound) that will be used to fill the crack. Workers will then break the shaft's ceiling and insert the polymer in a different spot from where they tried to place the concrete, the utility's officials said.

Late Sunday morning, Hidehiko Nishiyama of Japan's nuclear and industrial safety agency said work had yet to start on this phase of the operation -- though he did add those authorities have theories on where the tainted water came from originally.

Urgently addressing the problem is critical for two chief reasons: to ideally address recent alarmingly high levels of radiation spotted in seawater off the plant and to help determine where the radioactive material came from in the first place, with one expert suggesting it likely stems from at least the partial meltdown of nuclear fuel rods in a reactor core.

Radiation levels in the affected shaft itself measured at more than 1,000 millisieverts per hour, which is more than 330 times the dose an average resident of an industrialized country naturally receives in a year. Above, the level was 250 millisieverts per hour. The shaft lies at the end of a long channel that has been filling up for days.

This tainted water could at least partly explain readings, announced last Thursday based on samples of seawater taken 330 meters (361 yards) offshore, showing levels of iodine-131 at 4,385 times above the standard and cesium-137 at 527 times beyond normal. Experts say the latter radioactive isotope may be a greater concern because it persists longer, since it takes 30 years to lose half its radiation -- compared to an eight-day half-life for the iodine-131 isotope.

Plugging the external leak is job one. But it may not be the most difficult, or important, task ahead.

The ratio of the two isotopes in the seawater samples, combined with the discovery of the cracked shaft itself, supports the idea that the radioactivity is coming from the reactor and not the spent fuel pools at the plant, said Gary Was, a professor of nuclear engineering at the University of Michigan.

Finding how radioactive water got out of the reactor itself promises to be "exceptionally challenging," said physicist James Acton, with the Washington-based think tank the Carnegie Endowment.

To do so, officials must inspect complicated array of pipes inside the dangerous radioactive environment that now exists within the containment building, according to Acton, familiar with Japanese nuclear plants in part from having examined one another rocked by a 6.8-magnitude earthquake in 2007.

This struggle is part of the larger, daunting challenge facing Tokyo Electric and Japanese officials as they come to grips with the scope of the disaster and also work feverishly to keep nuclear fuel cool and prevent the further release of dangerously radioactive material.

The human cost became starkly apparent Sunday, when a power company official announced two workers missing since the 9.0-magnitude March 11 quake were found dead in the basement of the No. 4 reactor's turbine building. Both men, ages 21 and 24, appeared to have suffered multiple traumatic injuries. Their remains were found Wednesday.

More than a dozen others have been reported injured over the past three weeks, most of them tied to explosions in the Nos. 1 and 3 reactor complexes tied to the buildup of hydrogen. To prevent this from happening again, Nishiyama said there is a plan to inject nonflammable nitrogen into the Nos.1, 2 and 3 reactors to prevent another such hydrogen blast.

A buildup of hydrogen is an early sign of damage to a reactor's superheated core. But Nishiyama said no alarms had been sounded about rising pressure and that adding nitrogen would not force engineers to release hydrogen from the reactors.

Meanwhile, efforts continue to assess the amount and impact of radiation that has already gotten out.

That includes the spraying an experimental synthetic resin to lock in radioactive material on the ground, so it can't interfere with restoring cooling systems aimed at preventing the overheating of nuclear fuel in reactors and spent fuel pools.

Also, Jeff Immelt, the chief executive of General Electric Co., was heading to Japan to meet with Tokyo Electric authorities about stabilizing the damaged reactors, company spokeswoman Deidre Latour said Saturday. The reactors were designed by GE.

CNN's Tsukushi Ikeda, Yoko Wakatsuki, Junko Ogura, Midori Nakata and Susan Olson contributed to this report.