As thousands gathered on Thursday to mark the end of a 40-day mourning period for the former prime minister, Pakistani authorities announced two "important arrests" in connection with her assassination.
A senior police officer in Rawalpindi identified the suspects as Hasnain and Rifaqat, but gave no other details.
Two other people, including a 15-year-old who admitted being a backup suicide bomber, were arrested last month.
Cause of death
Controversy has raged in Pakistan over the cause of Bhutto's death - whether the late politician was killed by a bullet or by a head injury caused by the blast.
After the assassination, government officials asserted that Bhutto died by hitting her head against a sunroof handle in the car she was travelling in, but her political party and many Pakistanis did not accept that explanation.
A poll conducted by Gallup Pakistan found almost half of all Pakistanis believed government agencies or politicians allied to Pervez Musharraf, Pakistan's president, were involved in the assassination.
Musharraf asked Britain's Scotland Yard to help in the investigation.
According to The New York Times, the Scotland Yard inquiry has established that a single attacker fired on Bhutto and subsequently detonated explosives.
Government officials in Pakistan initially said there were two assailants.
The report comes less than two weeks before Pakistan's parliamentary elections on February 18, which were delayed by Bhutto's death.