SEOUL/CHEONAN -- The election campaign bus involved in the deaths of two campaign workers for presidential candidate Ahn Cheol-soo turned out to be illegally modified without the authorities’ approval, transportation ministry officials said Wednesday.

The workers, a local campaign chief and a bus driver, were found unconscious inside a campaign bus in Cheonan, about 90 kilometers south of Seoul, Tuesday afternoon and later pronounced dead at hospitals.

The candidate of the minor opposition People‘s Party suspended all campaign activities following the accident, which occurred on the day the 22-day official campaign kicked off for the March 9 presidential election.

The bus was equipped with an LED screen and power generator on the exterior. Police suspect carbon monoxide gas emitted from the equipment may be responsible for the deaths.

Ahn’s camp did not apply for approval from the Korea Transportation Safety Authority, required to install such devices, ministry officials said.

Police, as well as transportation and labor ministries, are probing the exact cause of the accident.

The police suspect the power generator, which supplies electricity to the screen, may have produced toxic carbon monoxide and suffocated the victims inside the bus. Officials reportedly plan to question employees at the LED screen company and how much they were aware of the danger of carbon monoxide poisoning.

A similar incident involving an Ahn campaign bus was also reported in the eastern city of Wonju on Tuesday, leaving one person in grave condition.

Ahn‘s party said it has been operating 18 similar campaign buses across the country.

Ahn paid his respects to the late campaign chief at a funeral home set up at Dankuk University Hospital in Cheonan on Tuesday and to the deceased bus driver at the nearby Soon Chun Hyang University Hospital in the city.

He declined to answer questions from reporters at the hospitals.

Yoon Suk-yeol, the presidential candidate of the main opposition People Power Party (PPP), visited the two hospitals later Wednesday to pay his respects, a campaign official said.

Lee Jae-myung of the liberal Democratic Party (DP) and Sim Sang-jeung of the progressive Justice Party also expressed their condolences during their campaign rallies.

The parties refrained from playing songs and dancing during campaign rallies Wednesday. (Yonhap)