The Pheu Thai Party has denied claims that the government of Yingluck Shinawatra ordered an investigation into "men in black" to be shelved after coming to power.
Cherdchai Tantisirin, a co-leader of the United Front for Democracy Against Dictatorship, said on Sunday the administration at the time did not intervene in the Department of Special Investigation (DSI) and order it to drop the probe into links between armed men and the attacks in 2010.
He also denied DSI investigators on the case were replaced.
He rejected reports of an unnamed source at the DSI saying the probe into "men in black" in connection with the events of April 10, 2010, was suspended after Pheu Thai took over the administration in 2011. The DSI was ordered to concentrate on the bloody crackdown on the red shirts a month later instead.
A "powerful politician" in the since-deposed government laid out a guideline for the DSI that the so-called men in black did not exist and there was no armed element, the source alleged.
"The reports lack credibility," the former Pheu Thai MP and red-shirt leader said. "It was the same old issue used to damage [Pheu Thai]," he added.
The "powerful politician" was believed to be Chalerm Yubamrung, who was deputy prime minister controlling police in the government led by then-prime minister Yingluck before a reshuffle saw him moved to the Labour Ministry.
But Dr Cherdchai said Mr Chalerm could not influence authorities and force them to shelve the investigation.
The DSI was commanded by Tarit Pengdith at the time. He was moved to an inactive position after the coup.
The issue came to light after police arrested four men and one woman on Wednesday over M79 and rifle attacks at Khok Wua intersection on April 10, 2010 that killed soldiers and civilians including Gen Romklao Thuvatham, who was an army colonel at the time.
Kittisak Soomsri, 45, Chamnan Phakeechai, 45, Preecha Yuyen, 24, Ronnarit Suricha, 33, and a 39-year-old woman, Punika Chusri, are detained at the Bangkok Remand Prison for further interrogation.
Prison chief Ayut Sinthopphan said on Sunday the suspects were under tight security and monitored by closed circuit cameras around the clock.
Lt Gen Nanthadet Meksawat, who was a member of a Senate subcommittee that looked into the "men in black" case, wrote on his Facebook page that the DSI under Mr Tarit would have arrested those involved in the April violence because they had the same information as police who apprehended the five on Wednesday.
"Had DSI director-general [Tarit] taken the investigation seriously, this issue should have come to an end a long time ago," he said.