A journalist was beaten on Wednesday during an undercover investigation into untreated water discharges by a paper mill.

The case raised nationwide concern over the protection of reporters' rights.

Zhao Xi, a reporter at China Central Television's bureau in Hunan province, was hit on the forehead by a man using a flashlight at about midnight on Wednesday.

At the time, Zhao and his co-workers had just found the water release outlet of Baiyang Paper Mill in Yueyang, Hunan province.

They were about to fetch equipment to take water samples when two men stopped them. Soon, a dozen more people came, saying they were workers at the mill. Zhao's glasses fell to the ground and his forehead started bleeding as he tried to protect the video camera.

Zhao said the man who hit him was in his 50s.

"He was very unfriendly from the very beginning. But I never expected an elderly man would hit me so hard," Zhao said.

Authorities in Pingjiang county have apologized to Zhao and detained two men: the man in his 50s and a younger man who threatened to throw Zhao's group and their car into the Miluo River. The river's pollution level and water quality were the subject of Zhao's investigation.

Zhao was taken to a hospital in the county and placed on an intravenous drip, and is expected to be discharged on Friday.

Authorities have launched investigations into the assault of the reporter, the environmental violations by the paper mill, and the delayed appearance of police, as officers appeared one hour after Zhao called for help, Zhao said.

With an annual output of 8,000 metric tons, the paper mill's license has been revoked. Yet up to Tuesday, the mill's internal records showed it continued to operate, a practice considered illegal by lawyers because the mill had already lost the license to carry out further production.

It is not the first time Zhao has met hostile people sabotaging his reporting. He and senior lawyers from the All-China Environment Federation, a nonprofit civil society organization, had started planning for the undercover trip to Pingjiang county since early December.

They arrived days ago, but had to track the outlet at night because mills disregarding water treatment usually make the discharges secretly in darkness.

The light from Zhao's filming equipment alerted people at the mill.

With nearly 10 years of experience as an investigative reporter, Zhao has had previous confrontations when he tried to uncover behind-the-scenes deals on high-profile real estate projects, corporate refusal to pay employees' pension insurance, and the incompetence of emergency medical care facilities.

To avoid unnecessary hostility, he does a thorough background check of his subjects, and takes safety precautions when he stays in hotel rooms during trips.

Despite all these measures, Zhao said he was shocked people can treat a reporter with such violence.

"Reporters out in the field doing investigations are very vulnerable. They need more forceful and immediate protection from the authorities, in my case, from the police, in emergencies, " he said.

Zhao said he will return to work and continue to focus on environmental topics.

"A reporter's safety should comes first, before the delivery of worthy news," he said.

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