Prosecutors sent a summons to Park earlier this week to appear for an interrogation on Tuesday, just 11 days after the Constitutional Court removed her from office on March 10.
"We are creating a questionnaire," a senior prosecutor said Friday, adding it is yet unclear how long it will take to complete the inquiry.
Ahead of the rare interrogation, prosecutors intensified their probe, summoning a number of company executives and government officials embroiled in the scandal.
Three former and incumbent executives of SK Group were quizzed Thursday over bribery allegations in connection with Park's close friend Choi Soon-sil.
Korean political commentator Sung Sik Seo analyses.
