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As Brian Fallon, Hillary Rodham Clinton’s press secretary, defended her on Wednesday during a conference call about her email use, he made a novel argument that got lost in the haze of questions.

Mr. Fallon spoke to reporters after Fox News had reported that two of Mrs. Clinton’s aides while she was secretary of state had sent her classified information over her private email server.

Mr. Fallon insisted that the report constituted a “watershed” moment that helped identify which emails an inspector general had flagged as containing classified information. That’s because, he argued, the definition of what is classified is subjective, and the emails weren’t marked as classified at the time they were sent. The inspector general referred to four classified emails in a letter to the F.B.I. about the security of Mrs. Clinton’s server.

Then Mr. Fallon tried to turn the theoretical tables on Representative Trey Gowdy of South Carolina, the chairman of the House select committee investigating the Benghazi attacks, which has focused in recent months on Mrs. Clinton’s email use. If she was at fault for having classified information on her server without her knowledge, Mr. Fallon suggested, then aren’t other people in the same boat?

“Just as an aside, for the I.G. to now declare the material as classified, since it was provided by State to the House Benghazi committee earlier this year in unredacted form, presumably that means that members of the House Benghazi committee may have unwittingly handled classified material on unclassified systems within the House of Representatives,” Mr. Fallon said.

“Now, I don’t think that anybody here at the Clinton campaign is going to say that members of, say, Chairman Gowdy’s staff should have their computers confiscated for having possibly trafficked in classified material,” he said. “I don’t think we would say that. But that is, fundamentally, the same logic behind the I.G.’s referral to the State Department with respect to Mrs. Clinton’s server, since she was at worst a passive recipient of unwitting information that subsequently became deemed as classified. Let’s raise that as an aside.”

On Thursday afternoon, a spokeswoman for the House committee said that “The Committee has taken a year to complete all of the complex procedures required by the Executive Branch for review and certification of its systems to manage and handle classified information in electronic form. Despite the substantial complication and inconvenience to complete all of the necessary requirements, the most important thing was to make sure we did it right.”

A spokesman for the Democrats on the committee, who make up the minority, said in an email that the documents the committee received were not marked classified, adding, “Like Secretary Clinton, Committee members and staff could not have known to treat the documents as classified when we received them, because it was not marked or easily identifiable as classified information.”

When the State Department changed one document to classified, he said, the committee moved it to a new system.

But Mr. Fallon’s comments represent a new argument about Mrs. Clinton’s email use at the State Department, which has continued to be a focus of attention. She has argued that she wants all of her emails to be public, which is why the State Department is going through them.

There have been a string of lawsuits over stalled freedom of information requests related to her emails, some coming from news media outlets and others from conservative groups, such as Citizens United.