TOKYO—A team of experts from the International Atomic Energy Agency began its investigation into the Fukushima Daiichi nuclear plant accident on Tuesday, amid allegations that slow decision making by plant operators and regulators may have exacerbated what has become the world's worst nuclear accident since the 1986 Chernobyl disaster.

The 18-member team, which is made up of six IAEA officials and 12 experts from various countries, will hold hearings with Japanese regulators and plant operator Tokyo Electric Power Co. to investigate the accident triggered by the March 11 magnitude-9 earthquake and ensuing tsunami.

The team, led by Michael Weightman, chief nuclear inspector of Britain, includes experts from the U.S., China, Russia and South Korea. The mission's purpose is "to learn lessons so we can improve nuclear safety...and nuclear trust around the world," Mr. Weightman said to reporters.

The investigation comes as opposition politicians say Prime Minister Naoto Kan's office appeared to have meddled in the recovery operation. At issue are apparent conflicts between Tepco and authorities over whether the prime minister's office ordered the operator to stop injecting seawater into overheating reactors amid safety concerns, a contention Mr. Kan has denied.

In its 10-day trip, the team is scheduled to visit the Fukushima Daiichi plant and the nearby Daini plant, which avoided any major problems in safely shutting down its four reactors.