In line with the constitutional convention that the prime minister must sit in the House of Representatives, he resigned from the Senate on 1 February to contest the by-election caused by Holt's death, which was held on 24 February. He won an easy victory, and was sworn in to the House on 12 March. According to his biographer, Tom Frame, "there could never realistically be much doubt that Harold Holt drowned – he was simply one of the number of ordinary Australians who drown each year through poor judgment or bad luck." Holt likely misjudged his own swimming ability and the roughness of the conditions, and was simply overcome by exhaustion. Alternatively, he may have suffered a heart attack, been struck by driftwood, stung by jellyfish, or attacked by a shark. Holt's body was probably either trapped below the surface or washed out to sea on the ebb tide. It was not unusual for this to occur – three men had drowned at a beach in Rye a few years earlier, with one body disappearing and the other two ending up in different places. Sir Robert Southey, a senior figure in the Liberal Party's organisational wing, said of the events in a 1994 interview:  Some have suggested that Holt entered the water primarily in order to impress Marjorie Gillespie, with whom he was rumoured to be having an affair. Zara believed that this was the case, and in 1988 Gillespie publicly identified herself as Holt's lover. However, in an earlier interview she had been specifically asked if their relationship was sexual in nature, and did not characterise it as such. The Victoria Police launched a formal investigation into Holt's disappearance the day after it occurred. Jack Ford, a former homicide detective, was chosen to head the investigation, with Aubrey Jackson of the Commonwealth Police assisting. The resulting police report was released on 5 January 1968, but did not record any definitive findings due to a lack of evidence. Senior pathologist James McNamara was consulted about what might have happened to Holt's body, and suggested that it may have been trapped by kelp and then consumed by sea creatures (specifically sharks, crayfish, and/or sea lice). If that were the case, "the body would have been reduced to a skeleton in a period as short as 24 to 48 hours". Some of those involved in the investigation later reported that certain relevant information had been deliberately omitted from the final report – for instance, Simpson's statement that Holt had had several cans of beer in his bag. The federal government declined to conduct its own inquiry, as the disappearance was considered uncontroversial and his family did not want one.