▶ Watch Video: Mike Pence won’t fight subpoena to testify in special counsel probe

A handful of current and former U.S. Secret Service officials are set to testify on Friday before a Washington, D.C., grand jury as part of the special counsel investigation into last year’s FBI search at former President Donald Trump’s Mar-a-Lago estate in which classified documents were seized, a source familiar with the ongoing investigation told CBS News.

Multiple current and multiple former USSS officials are expected to testify, the source said, declining to provide an exact number.

The grand jury proceedings are part of special counsel Jack Smith’s investigation into the Mar-a-Lago search. Last November, Attorney General Merrick Garland appointed Smith to oversee the Justice Department’s investigation into the Mar-a-Lago search, along with the agency’s investigation into Trump’s role in the Jan. 6 Capitol attack.

Last week, a judge ordered Trump attorney Evan Corcoran to also testify before the grand jury in connection with the Aug. 8 Mar-a-Lago search in which the FBI seized 33 boxes, including dozens of documents with classified markings that were supposed to have been handed over to the National Archives.  

The source told CBS News that this is believed to be the first appearance of USSS officials, either current or former, before the D.C. grand jury, but stressed it is likely not the last.

Several other USSS officials will be slated to appear in the coming weeks, the source said, and some have not yet been approached by the FBI.

CBS News has reached out to USSS for comment. Friday’s scheduled testimony was first reported by Fox News.