MANILA - Senator Alan Peter Cayetano insinuated Thursday that the Liberal Party (LP) may be behind a witness who linked President Duterte to vigilante killings of the alleged Davao Death Squad (DDS).

Cayetano also traded barbs with LP's Senator Leila de Lima, as he assailed the credibility of witness Edgar Matobato, a self-confessed DDS hitman.

Cayetano said the Senate should verify Matobato's background as he may be part of LP's alleged "plan B" to install Vice President Robredo in Malacañang.

"I am testing his credibility because I'm asking about you (de Lima), your motive, the motive of your party in this hearing. I am testing whether he is a credible witness or this is part of the plan B of the Liberal Party na makuha ang [to steal] Malacañang," Cayetano said.

READ: Robredo: Witness' allegations vs. Duterte 'serious'

Matobato, for his part, said he is not part of any plot to bring down the government.

"Hindi man ako nagsisira kay President Duterte, sir. Sinasabi ko lang ang totoong-totoo kung ano talaga ang tinatrabaho namin," he said.
[I am not trying to destroy President Duterte. I'm merely saying the truth about our job.]

"Basta ang sinasabi ko ay iyung inuutos lang sa amin ni President Duterte... Walang nag-utos sa akin na tao," he added.

Unsatisfied, Cayetano pressed Matoabato on who helped him hide from the DDS and how he eventually ended up as a Senate witness.

At this point, De Lima interrupted Cayetano's line of questioning, saying that Matobato may not be able to give truthful answers out of fear for his safety and those who helped him.

She then proposed a closed-door session where the witness can answer Cayetano.

Cayetano, however, quipped that Matobato is having difficulty answering because he was lying. The senator added that an executive session would be unfair because Duterte was not given the same privilege.

"Bakit kapag laban kay President Duterte hindi muna ivini-vet, hindi muna executive session? Buong Pilipinas pwede itong marinig. Pero kapag tini-testing ko na ang kanyang credibility, hindi pwedeng marinig?" he said.

[Why is it that accusations against President Duterte are not vetted first through an executive session. The whole Philippines can hear the accusation. But now that I'm testing the witness' credibility, it can't be heard by the public?]

Senator Antonio Trillanes IV then threatened to cite Cayetano for being "out of order" over his prolonged questioning.

WATCH: Cayetano, Trillanes clash in Senate hearing

De Lima called on a two-minute break to diffuse tensions in the hearing, after which Cayetano took some 20 more minutes questioning Matabato.

When de Lima asked Cayetano to wrap up, he insisted: "We have all the time in the world. Ano bang mas importante, lumabas ang totoo [What is more important, for the truth to come out] or is this just a demolition job against Duterte so we don't care?"

A bristling de Lima then declared Cayetano out of order for insinuating that the investigation was biased.

"It is unparliamentary for a member of the Senate to be imputing certain ill motives on the part of the chair or the standing committee. You have been doing that, you have been insinuating things indicative of bad faith, ill motive on the part of the standing committee, including the chair. I am not going to further allow that," she said.

Cayetano attempted to talk over de Lima but the Senate's sergeant-at-arms muted his microphone.