The explosion happened early on Tuesday at Zubao Coal Mine in Lianyuan, a county-level city in Loudi.
Mine authorities reported that of the 29 people who were working in the pit, 16 escaped the shaft, nine died and four were rescued, state-run Xinhua news agency reported.
On Thursday, investigators from the city government of Loudi found one more miner had been killed in the blast, but was listed as "injured", the Loudi government said in a statement.
Three officials are under investigation, including two deputy mayors of Lianyuan and the local production safety chief.
Hunan Province has launched a two-week safety overhaul and suspended all small coal mines with annual production capacity of less than 300,000 tonnes.
Zubao Coal Mine is a small mine with an annual capacity of 90,000 tonnes.