Christina Bobb told justice department on Friday that she signed supposedly complete list of documents at direction of another lawyer, Evan Corcoran

Donald Trump’s lawyer Christina Bobb was instructed to certify to the justice department that all sensitive government documents stored at his Mar-a-Lago resort subpoenaed by a grand jury had been returned, though she had not herself conducted the search for the records.

The certification of subpoena compliance appears to be at the center of a criminal investigation into obstruction of justice surrounding the former US president after the assurance was proved to be untrue when the FBI’s search of the property turned up more than 100 more documents marked classified.

The saga around the Mar-a-Lago documents has become increasingly fraught in the courts as Trump has repeatedly claimed the FBI search and resultant investigation is politically motivated. Meanwhile, the justice department and Democrats have portrayed the taking of the documents as a potentially serious national security breach.

Bobb signed the certification as the “custodian of records” at the direction of another Trump lawyer, Evan Corcoran, and only later added caveats to make the declaration less ironclad since she had not conducted the search herself, according to three sources familiar with the matter.

The certification was drafted by Corcoran, who also searched Mar-a-Lago for documents demanded by the subpoena, and sent it to Bobb before the justice department’s counterintelligence chief, Jay Bratt, arrived on 3 June to collect a folder of responsive records, the sources said.

But unsure as to whether the subpoena had been fully complied with, Bobb told Corcoran to amend the certification to say that “based upon the information that has been provided to me” all documents responsive to the subpoena were being returned after a “diligent” search, the sources said.

It was not clear why Bobb was willing to sign the declaration – as required by the subpoena in lieu of testimony – as the “custodian of records” when she never fulfilled such a role, the sources said, and appeared to know there was risk in attesting to a search she had not completed.

And it was also not clear why Corcoran, who had been liaising with the justice department for weeks over government records at Mar-a-Lago, according to court filings, did not himself sign the certification – as well as whether he had asked for Bobb’s signature.

Bobb testified to the justice department about the 3 June episode on Friday, detailing Corcoran’s role and additional contacts with Trump’s in-house counsel Boris Epshteyn, one of the sources said. NBC News earlier reported Bobb’s testimony.

The circumstance surrounding the subpoena certification has become of special interest to the justice department as it conducts a criminal investigation into possible violations of the Espionage Act, obstruction of justice and the retention of government records.

At issue for the obstruction line of inquiry, according to the partially redacted search warrant affidavit and recent court filings, is whether the letter signed by Bobb was wilful misrepresentation so Trump could hide other documents marked classified at Mar-a-Lago.

“Efforts were likely taken to obstruct the government’s investigation,” federal prosecutors alleged in an August court filing.

But the previously unreported details behind the certification – and accepted as sufficient by the justice department on 3 June – could change the dynamics of the obstruction investigation and minimise Bobb’s potential legal exposure if she was misinformed, former justice department officials said.

The justice department appears to view Bobb and Corcoran as witnesses instead of potential defendants, even if that can change without warning: in a motion to the US appeals court for the 11th circuit, prosecutors wrote that Trump’s lawyers might be “witnesses to relevant events”.

Corcoran did not respond to requests for comment. Bobb and her criminal defense attorney also did not respond to requests for comment, though Bobb has told associates since the FBI’s search of the property on 8 August that the certification she signed was truthful, the sources said.

Bobb is considered as in-house counsel on Trump’s legal team, but was not involved in the Mar-a-Lago documents case in the run-up to the FBI search of the property in Florida and signed the certification as the “custodian of records” in name only, the sources said.

The distinction is important: in signing the statement as a custodian rather than as a lawyer, Bobb might not be subject to attorney-client privilege protections and thus could be able to speak more freely.