New Delhi: Delhi Police is set to expand its probe into the 2008 'cash-for-vote' scam and could possibly question Rajya Sabha MP Amar Singh whose  "aide" Sanjeev Saxena was arrested in connection with the case.

The investigators have sent a letter to the Union Home Ministry in which they are understood to have sought its views and directions to question Singh and an another MP.

"We have sent a letter to the Home Ministry," a senior police official said.

Under the law, permission is required from Rajya Sabha Chairman or Lok Sabha Speaker if the parliamentarians are to be questioned.

Saxena, who is said to be close to Singh and allegedly carried the cash to the house of 3 BJP MPs to ensure that they vote in favour of UPA government in the 2008 trust vote, was arrested on Tuesday.

Police is also looking for a driver, Sanjay, who ferried Saxena to the houses of the MPs and a mobile phone used by him to make calls while delivering the cash.

Allegations were levelled by the then BJP MPs Ashok Argal, Faggan Singh Kulaste and Mahavir Bhagora that Saxena had delivered Rs one crore in cash to them at the 4 Ferozshah Road residence of Ashok Argal to vote in favour of government.

The crime branch submitted two out of three BJP MPs -- Faggan Singh Kulaste and Mahavir Bhagora -- have stated that on July 22, 2008, Saxena delivered money to them and third MP Ashok Argal at the latter's residence here.

Kulaste has alleged Saxena was introduced to him and Argal by Amar Singh as his secretary on July 22, 2008, according to the prosecution.

The case was registered in 2009 on the recommendation of the Parliamentary Committee. Saxena has been booked for various offences under the Prevention of Corruption Act, police said.