A well-known instructor who teaches the U.S. standardized Scholastic Aptitude Test (SAT) was kidnapped and was forced to sign a renewal contract, amid the widening police investigation into the recent leakage of questions of the test, the Hankook Ilbo said Saturday.

The authorities said the owner of a private learning institute in Seoul specializing in preparing students for SAT, who was arrested Monday for leaking questions to Koreans in the United States, was also discovered to have kidnapped and then assaulted a colleague, forcing him to sign a renewal contract.

According to the police, the owner kidnapped the well-known SAT writing instructor, identified only as B, 38, took him to a resort villa on the outskirts of Seoul. He beat B several times, threatened with a knife and forced him to sign a renewal contract.

After gaining freedom, B reported the case to the police.

B taught the writing section of SAT and was known to be a sought-after figure among students. A fee to sit in on his course for a month as much as costs several million won, according to the report.

As SAT has gained popularity in Korea with the increasing number of Koreans students who choose to study in the United States, competition among private institutions in Seoul to recruit instructors with good reputation has become fierce, sometimes creating legal disputes when an instructor wants to transfer to another competitor institution.

"Students choose an institution based on a particular figure, who is known to teach well, creating a ferocious competition among private institutions to recruit them. These figures are often paid hundreds of millions of won annually," said an identified person who knows the situation well, in the article.

The news comes at a time when Korean police are expanding their investigation of private academic institutes in Gangnam, southern Seoul.

An instructor identified by his surname, Jang, and three college students, who were temporarily hired by the instructor, are now being questioned for smuggling SAT exam sheets out from a test center in Gapyeong, Gyeonggi Province, where they took the test last Saturday.

Going to U.S. universities has become popular among those who are not satisfied with the level of education offered by Korean universities, as well as those who cannot get into the most competitive Korean universities.

In 2008, South Korean households spent 20.9 trillion won ($18 billion) on private education to supplement the perceived shortcomings of the public school system, and the number of private educational institutes has increased nearly 50-fold since 1970, according to the education ministry.