Officials at the governing body asked the Ibrox club to accept title-stripping as a condition of being granted SFA membership last summer.
New Gers chief executive Charles Green rejected the request – and the findings of an independent commission yesterday backed up his decision.
The SPL commission ruled no unfair sporting advantage had been gained by side-letter arrangements.
Smith, the former chief executive of the SFA, said the identity of the person who asked the Glasgow giants to accept losing titles needs to be ascertained.
He said: "Rangers, in order to retain their place in Scottish football, were asked to surrender their titles. At that stage, it was almost an admission of guilt
"It is something that will now need to be investigated. We need to see who the people were who forced that issue.
"I was not surprised to see Charles Green raise that issue very quickly yesterday and I think it is something that needs to be looked at further."
Smith defended SPL chief executive Neil Doncaster over his handling of the affair.
Smith added: "The SPL is run by the committee.
"If they wanted an investigation, there is not much the chief executive could have done differently."