TOKYO - Japan's labor ministry referred advertising agency Dentsu Inc. and one of its executives to prosecutors on Wednesday over an alleged overwork-related suicide of a 24-year-old employee, saying that the case amounts to an indictment.

Labor authorities suspect the Japanese advertising giant and the employee's supervisor violated the Labor Standards Law by forcing Matsuri Takahashi and another employee to work illegally long hours between October and December 2015. Takahashi killed herself in Tokyo on Christmas Day last year.

The latest move by the Health, Labor and Welfare Ministry's Tokyo Labor Bureau came unusually fast after the ministry raided Dentsu on Nov. 7.

At Dentsu, over 30 employees were found to have been forced to work excessive and unlawful amounts of overtime, which they underreported by more than 100 hours per month. The ministry will continue its probe on the possible involvement of other Dentsu executives, sources close to the matter said.

"We take the incident seriously," the company said in a statement. "We offer our apology to those concerned for causing such a situation."

An official of the Tokyo Labor Bureau said in a press conference Wednesday that it asked for "strict punishment" when sending the case to the prosecutors, meaning an indictment.

The bureau said it has already questioned Dentsu President Tadashi Ishii over the case.

Takahashi, who showed signs of depression prior to her death, was found to have worked overtime in excess of the maximum number of hours allowed under a labor-management agreement.

In a previous case in 1991, a 24-year-old Dentsu employee killed himself due to overwork, while the advertising agency's Chubu and Kansai branches as well as its Tokyo headquarters were acknowledged to have engaged in illegal overtime practices and had been advised by labor authorities from 2010 to 2015 to correct their conduct.

After joining Dentsu in April 2015, Takahashi jumped to her death from the upper floor of a company dormitory on Dec. 25 that year, according to a lawyer for her family.

The Tokyo labor standards inspection office in September this year recognized her suicide as a case of "karoshi," or death from overwork, as her overtime hours had significantly increased from some 40 hours a month to over 100 hours before she began to suffer from depression.

Her family argues she was also a victim of power harassment, with her boss allegedly telling her, "You have too little capacity (to handle work) if you find the current volume of work hard to handle."

In a statement, Takahashi's 53-year-old mother Yukimi called on the company to "make sure it implemented efforts to end long working hours so there will not be a victim like Matsuri in the future."

Japan's Labor Standards Act stipulates working hours must, in principle, not exceed 40 hours per week or eight hours per day. To have employees work longer, a labor-management agreement must be concluded beforehand.

A man in his 40s who left Dentsu several years ago urged company executives to think hard about how the agency is run.

"Looking back, the way I worked was insane," he said. "The management must reflect on their conduct by taking seriously the fact that it has led to suicides of employees due to overwork."