Those arrested include the operator of an illegal mine, Hou Zhenrun, who is suspected of ordering eight thugs to beat up two reporters on January 10 at the small mine outside the northern city of Datong.
Some media reports say that Lan and a colleague, Chang Hanwen, were attacked by up to 20 men as they were on their way to meet Hou.
Chang received a broken arm and bruises, the China Trade News said in a statement.
Police have said that Lan sought out Hou for a bribe, offering not to report on his illegal mining operation in exchange for money.
But the killing has highlighted the Communist party's tense relationship with the media, which the party strictly controls.