Tokyo (CNN) -- As Japanese Prime Minister Naoto Kan readied for a visit Saturday to the stricken Fukushima Daiichi nuclear plant, efforts continued to unravel the mystery as to how large amounts of radioactive material got into groundwater and seawater nearby and how to stop it.
Data from eight new monitoring posts around the plant showed that airborne radiation levels had stabilized, at between .390 and .0019 millisieverts per hour, a utility company official noted Saturday. This is far below the 400 millisievert-per-hour high reported between Units 3 and 4 on March 15, and also far shy of the 3 millisieverts that individuals in developed countries are naturally exposed to in a year.
Still, authorities don't have any explanation as to the source of recently reported high levels of radiation in water around the power plant, which is 240 kilometers (150 miles) north of Tokyo.
That includes a finding on seawater, derived from samples taken Wednesday afternoon at an offshore monitoring post 330 meters (361 yards) from the plant, showing levels of iodine-131 measuring 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.
On Friday, Tokyo Electric announced -- and later retracted, saying further testing and analysis was needed -- a reading of radiation 10,000 times above normal in groundwater near the No. 1 reactor complex.
That came days after Tokyo Electric officials announced that water in an exposed maintenance tunnel connected to the No. 2 unit's turbine building showed radioactivity at least 100,000 times normal levels for coolants inside a nuclear reactor.
High radiation levels were also found in tunnels leading to the Nos. 1 and 3 units. Authorities in recent days have reported some success in draining the water, such that there's no longer an urgent risk of it overflowing onto the surrounding ground.
Still, that does not mean they know where this radiation came from. To this end, a camera was to be installed in the trench leading to the No. 2 unit's turbine building to scope out the area, a Tokyo Electric official said early Saturday.
Also Saturday, spraying was set to continue of an experimental new material to lock in radioactive material in and around the nuclear complex -- so that it doesn't seep further into the air, water or ground.
Crews have dispersed about 2,000 liters (more than 500 gallons) of synthetic resin in a 500-square meter locale, according to Tokyo Electric. The aim is to hold the released radioactivity on the ground, so it can't interfere with the restoration of the cooling systems aimed at preventing the overheating of nuclear fuel rods in reactors and spent fuel pools at the plant.
"You spray it to hold down the loose contamination, and it acts like a super glue," said Nolan Hertel, a radiation engineering expert at the Georgia Institute of Technology in Atlanta. "You don't want radiaoactive materials that are loose to get away."
Meanwhile, no new information on water radiation levels were released Saturday at a press briefing of Japan's nuclear and industrial safety agency. Officials explained that -- after releasing such information sometimes twice a day earlier this week -- they are reevaluating the current and present data to ensure if veracity.
The back-and-forth about potentially dangerous radiation levels became apparent again Friday, as Japan's health ministry announced there is no radioactive contamination of beef in the beleaguered country -- describing an earlier, contrary report as wrong.
The ministry said faulty testing may have led to the first result, which indicated levels of radiation slightly in excess of those allowed by Japanese law. In a second round of tests, lab workers were unable to find any trace of radioactive cesium in the same cattle, according to the reports.
But a second examination of the same sample of beef confirmed the negative results to the health ministry.
Representatives from the town where the beef was from -- Tenei-mura, Fukushima -- have sent a written letter of protest to the governor, according to a report in the Asahi Shimbun newspaper.
"Was it necessary to release the first test result even though they knew they were conducting a second test?" questioned Tsukasa Kaneko, the chief of the village, according to the newspaper.
"To stop the release of early unconfirmed reports is the duty of the government, but now they have enflamed the situation and caused fear," the newspaper quoted him as saying.
CNN's Yoko Wakatsuki, Junko Ogura and Rich Phillips contributed to this report.