Senator Somchai Swangkarn on Tuesday called on the prime minister to order an investigation into the early release of former Democrat Party MP Khanchit Thapsuwan, who was freed from jail after almost eight years for masterminding a murder in 2011.
The Supreme Court in December 2016 upheld a life sentence handed down to Khanchit for the premeditated murder in 2011 of Udon Kraiwatnusorn, a former president of the Samut Sakhon provincial administrative organisation.
Udon was murdered on Dec 25, 2011 at a PTT petrol station near kilometre marker No.2 on Setthakij Road in tambon Tha Sai in Muang district of Samut Sakhon.
He was washing his hands in the toilet when the killer approached and shot him 10 times with a .40 calibre handgun. He died at the scene.
The provincial court on Nov 12, 2014 sentenced Khanchit to death for premeditated murder. In October 2015, the Appeal Court reduced the sentence to life imprisonment. Prosecutors then forwarded the case to the Supreme Court.
The Supreme Court later upheld the life sentence handed down by the Appeal Court and ordered Khanchit to pay 13.3 million baht in compensation to Udon's family.
In all, Khanchit spent seven years, 10 months and 25 days in jail before he was released from Bangkwang Central Prison on Oct 3.
On Tuesday, Mr Somchai, in his capacity as chairman of the Senate committee on human rights, said on his Facebook page that checks showed possible irregularities in Khanchit's reduced terms.
This has raised public doubt as to whether graft was at play.
"Can the Corrections Department act as if it were the court to determine the fate of the inmate?" Mr Somchai wrote. "The Supreme Court upheld the life sentence handed down to the convict, but his sentence was commuted from 43 years to seven years."
The senator urged Gen Prayut Chan-o-cha to instruct related agencies to carry out the probe.