TOKYO – A radiation alarm went off at Tokyo Electric Power Co.’s Fukushima nuclear power plant before the tsunami hit on March 11, suggesting that contrary to earlier assumptions the reactors were damaged by the earthquake that spawned the wall of water.

A monitoring post on the perimeter of the plant about 1 mile from the No. 1 reactor went off at 3:29 p.m., minutes before the station was overwhelmed by the tsunami that knocked out backup power that kept reactor cooling systems running, according to documents supplied by the company. The monitor was set to go off at high levels of radiation, an official said.

“We are still investigating whether the monitoring post was working properly,” said Teruaki Kobayashi, the company’s head of nuclear facility management. “There is a possibility that radiation leaked before the tsunami arrived.” Kobayashi said he didn’t have the exact radiation reading that would trigger the sensor.

Officials at the company, known as Tepco, had earlier said the plant stood up to the magnitude-9 quake and was crippled by the tsunami that followed, causing the world’s worst nuclear crisis since Chernobyl in 1986. The early radiation alarm has implications for other reactors in Japan, one of the most earthquake prone countries in the world, because safety upgrades ordered by the government since March 11 have focused on the threat from tsunamis.

Many scientists have considered the possibility of damage to Fukushima reactors from the quake, said Tetsuo Ito, head of the Atomic Energy Research Institute at Kinki University in western Japan. “Utilities should reinforce safety measures at nuclear power plants, particularly ones housing old reactors like the Fukushima station.”

Tepco this week released thousands of pages of documents that highlight the chaos in the early hours of the disaster as workers frantically tried to prevent meltdowns in three of six reactors at Fukushima. They included pictures of whiteboards with scribbled notes of times and events.