Police said Wednesday they launched an investigation against five protesting doctors affiliated with the Korea Medical Association, the country’s largest doctors’ group, for charges of breaching medical law.

Officials added that the case had been assigned to the public crime investigation division under the Seoul Metropolitan Police Agency.

The announcement came a day after the health ministry filed complaints against the five, marking the first instance of legal action taken against doctors protesting the government plan to expand the medical school student quota by 2,000 a year. The ministry accused the doctors of breaching Articles 59 and 88 of the Medical Service Act and Obstruction of Justice, as well as aiding and abetting junior doctors’ collective resignations.

The five doctors are Kim Taek-woo, who heads the emergency committee of the KMA; Joo Soo-ho, the head of the emergency committee’s public relations council; Park Myung-ha, the president of the Seoul Medical Association and the chairperson of the emergency body’s strengthening committee; Lim Hyun-taek, president of the Korean Pediatric Association; and Roh Hwan-kyu, KMA’s former chairperson. The KMA holds a membership of some 140,000 doctors across the country.

On the same day, officials also visited the homes of trainee doctors who left their jobs collectively last week to deliver back-to-work orders, in a move that would allow the government to file a criminal complaint over the labor action.

The move comes after junior doctors refused to receive the order through text messages by turning off their mobile phones. However, the document can be delivered to a representative, such as their cohabiting families, even when the recipient cannot be reached at the intended place of delivery under the Administrative Procedure Act.

The ministry has also requested cooperation from the police to be ready for any situation, according to officials.

On Monday, Interior Ministry Lee Sang-min issued an ultimatum for junior doctors which stated the government will not hold them accountable if they return to work by the end of this month, citing continued disruptions at medical institutions.

Second Vice Health Minister Park Min-soo also urged trainee doctors to call off their collective action, warning that those who refuse the back-to-work deadline by Thursday could face punishment the following day. Park noted that the suspension of doctors’ licenses for a minimum of three months would be unavoidable, adding that they could also face judicial measures such as investigations and prosecutions.

Meanwhile, 9,909 trainee doctors at 99 training hospitals had tendered their resignations as of Monday at 7 p.m., while 8,939 have left their workplaces, according to the Health Ministry.