The Office of the Attorney-General (OAG) has set up a panel to look into the decision of prosecutors to drop charges against Vorayuth "Boss" Yoovidhya, after the eight-year fatal hit-and-run case came to an end with the final allegation dropped by prosecutors.

Deputy OAG spokesman Prayuth Petchkhun said in a statement released on Sunday that Deputy Attorney-General Somsak Tiyawanich will lead a seven-member panel to investigate the decision of prosecutors to drop the charges against the Red Bull scion.

The case effectively ended when the Southern Bangkok Criminal Litigation Department decided in June to acquit him on an allegation of reckless driving that killed Pol Sgt Maj Wichian Klanprasert on Sept 3, 2012. The Royal Police Office (RPO) later agreed with the acquittal.

Mr Vorayuth's black Ferrari rammed into the police motorcycle on Sukhumvit Road. He then fled the scene to his house.

Sunday's statement was the first time that the OAG broke silence on the issue after the decision of prosecutors to drop the final charge was first reported on CNN on Thursday.

The working group has been asked to conduct the investigation quickly. It will probe the reasons for prosecutors not to pursue the charges and whether their decisions were in line with all laws and regulations, Mr Prayuth said.

"Mr Vorayuth can now return to Thailand without any problem. At this moment, we don’t know where he [Boss] is,” deputy police spokesman Pol Col Kissana Phathanacharoen said on Friday.

The OAG was under pressure to explain to the public why the case has been closed after years of legal inaction.