A professor at Nippon Dental University who allegedly leaked questions on a national dentistry examination to another professor had been doing so for two straight years, sources close to the case said Monday.

The same day, police searched the office of 56-year-old Motoo Niwa, who was arrested Sunday on suspicion of leaking questions on the 2000 national dentistry examination to former Ohu University professor Kenji Kusunoki and Itsuo Ueda, a professor at the Health Sciences University of Hokkaido.

Niwa's home, in Tokyo's Toshima Ward, was searched Sunday, police said.

Niwa, who served on the national dentistry exam board until June, is suspected of leaking key words to questions in the national exam held last March to Ueda by fax in November 1999, police said.

Ueda, 59, then passed the information on to Kusunoki, 54, they said.

Their collusion helped Kusunoki include three questions on oral hygiene in Ohu's exam given to final-year students on Feb. 17 and 18 last year, police said.

Ueda and Kusunoki have told police that they received similar assistance from Niwa the previous year as well, according to the sources.

Niwa has told police that he was not sure if he had mentioned the three questions that appeared in the Ohu exam to Ueda, according to investigative sources.

According to investigators, Niwa became acquainted with the other suspects at various gatherings of oral hygiene experts.

Kusunoki has told police that he asked Niwa for information regarding the national exams "on every occasion" they met, they added.

Ohu is a private university in Koriyama, Fukushima Prefecture, that has a graduate school of dentistry. Nippon Dental University is in Tokyo's Chiyoda Ward.