A progovernment newspaper in Cape Town reported yesterday that members of the ruling National Party are considering an investigation of South African opposition leader Helen Suzman's parliamentary initiative on the recent death of a political detainee.

On Tuesday, Suzman read into the record a letter charging that political detainee Neil Aggett was beaten by the security police. Dr. Aggett, a white organizer of a multiracial trade union, died in detention Feb. 5. The police say he hanged himself.

The speaker of the lower house agreed with government accusations that Suzman had deliberately broken a court ruling that prohibited discussion in Parliament of Aggett's death. When the minister of police challenged her to reveal the author of the letter, she replied: "I will if you promise not to beat him up, too."

Minister of Justice Kobie Coetsee said Suzman had proved she could "fabricate" documents. He promised a public inquest into Aggett's death.

In an interview, Suzman said she had obtained the letter from a lawyer, and it had been written by someone still in detention. She said she had deliberately not asked the author's name so that she could not be compelled to reveal it until she had obtained assurances that he would not be victimized. She said she could obtain the name if she wished.

Suzman denied fabricating the letter. She said she could not say whether it was authentic. She said the point was that its claims should be investigated: If she didn't know if they were true, Coetsee didn't know if they were untrue.