BEIJING — Chinese authorities have released all employees of a U.S. corporate due diligence firm detained in Beijing two years ago in an apparent move to reassure foreign firms operating in China amid sliding foreign investment in the world’s No. 2 economy.

The detained Beijing employees at Mintz Group, all Chinese nationals, have been freed, a company spokesperson told Reuters on Tuesday.

News of their release came on the heels of the just-concluded China Development Forum in…