South Korean prosecutors said Tuesday that they have sent out a summons to Woo Byung-woo, a key aide of ousted President Park Geun-hye, to appear for questioning regarding corruption allegations against the former leader.

Woo, 50, who served Park as a senior secretary for civil affairs from 2015 to 2016, is being asked to present himself at the Seoul Central District Prosecutors’ Office at 10 a.m. Thursday, according to an official from the prosecution’s special investigation unit.

The summoning signals the shifting focus of the investigation to Park’s former aide, who has remained largely unscathed so far despite having the nickname of “emperor” for his influence within the Park administration.

Woo has been questioned twice since the scandal involving the former president broke out in late 2016 -- once by state prosecutors and then by an independent counsel.

Special counsel Park Young-soo, whose 70-day investigation ended on Feb. 28, had seen Woo as a key figure in the corruption and influence-peddling scandal, suspecting that he may have abetted irregularities committed by Park’s longtime friend Choi Soon-sil.

Woo, however, has avoided legal action due to the special counsel’s limited investigation period, though a total of 30, including Choi, Samsung Group’s de-facto chief Lee Jae-yong and many of Park’s former aides, have been indicted.

Key allegations against him include one that he used his influence to block a prosecutorial probe into failed rescue efforts during the 2014 sinking of the Sewol ferry.

On Monday, prosecutors reportedly secured statements from a senior prosecutor at the Gwangju District Prosecutors’ Office who had led the investigation into the ferry tragedy, saying that Woo had interfered in the probe.

Woo is denying the allegation, although he admits to making a phone call to the prosecutor “to check on the status of the investigation.”

Former President Park’s inept response to the national tragedy was among the reasons behind the parliamentary vote to impeach her on Dec. 9.

On March 10, the Constitutional Court upheld the impeachment, saying she had undermined democracy and the Constitution by allowing Choi to meddle in state affairs and using her presidential authority to aid Choi’s private interests.

But it dismissed the charge regarding the ferry disaster, saying a lack of diligence was not grounds for impeachment.

Park was taken into custody Friday, 21 days after she was removed from office.

By Bak Se-hwan (sh@heraldcorp.com)