A senior police official said the Crime Branch team led by Deputy Commissioner of Police Ashok Chand grilled Rewati Raman for about 90 minutes while Argal will be questioned later this evening.
Argal will be the fifth person to be quizzed in the past 10 days after the Supreme Court came down heavily on investigators for carrying out a shoddy probe.
Rewati Raman, the 68-year-old Allahabad MP, said he told police what he told the Parliamentary panel which looked into the scam.
"I have told them what I have told the Parliamentary panel. The case is before Supreme Court now. I want the conspiracy behind the whole episode exposed," he told reporters.
The questioning of Rewati Raman comes three days after his former party colleague Amar Singh was questioned by the investigators. Two persons Sanjeev Saxena, former aide of Amar Singh, and Suhail Hindustani, an alleged go-between Amar Singh and BJP sought to be bribed, were arrested after quizzing.
The aim of questioning is to ask Rewati Raman about the telephone call he received from Hindustani on the midnight of July 21 and 22 to strike a deal to buy BJP MPs and to know why he went to the house of Argal as alleged by the other side, a senior police official said.
Police had already given a virtual clean chit to Rewati Raman and others saying it was Hindustani only who tried to approach them for striking a deal for MPs to vote in favour of the UPA government.