Commerce Department lawyers blocked a former official from answering almost 100 questions during a closed-door House Oversight hearing probing the origins of the Trump administration's decision to add a citizenship question to the 2020 census, according to documents released Tuesday.

The committee said in a memo it was unable to obtain some key information because department lawyers blocked James Uthmeier, a former senior adviser and counsel to Commerce Secretary Wilbur Ross, from answering questions about the rationale to add the question.

During his testimony, department lawyers instructed Uthmeier not to reveal his discussions with the White House about the citizenship question or reveal the identities of administration officials he spoke with about the question, citing executive branch confidentiality and litigation concerns, according to transcripts released by the committee.

The documents shed some additional light on the basis for adding the question, which the Supreme Court is set rule on this week.

"The Trump Administration claimed that the only reason it wanted to add the citizenship question was to help the Department of Justice enforce the Voting Rights Act, but that claim has now been exposed as a pretext," House Oversight chairman Elijah Cummings, D-Md., said in a statement.

"Official after official appearing before the Committee have refused to answer questions about the real reasons behind their effort, but the mounting evidence points to a partisan and discriminatory effort to harm the interests of Democrats and non-Whites," he added.

Ross argued during a House Oversight hearing in March that the Justice Department made a formal request in December 2017 for the Census Bureau to reinstate the citizenship question "for use in Voting Rights Act enforcement."

However, Uthmeier told the committee that within months of being sworn in, Ross asked him to look into the possibility of adding the question.

"The secretary asked me to look into the issue. I would say at some point in the spring of 2017, likely March or April," Uthmeier said, according to the transcript, adding that he discussed this issue with Ross "multiple times."

However, department lawyers also blocked Uthmeier from revealing the contents of what the committee called a "secret memo" he wrote about the question at Ross' direction and hand-delivered to John Gore, a Justice Department official.

Uthmeier also revealed that he sought guidance from John Baker, a law professor who authored the book "How to Get Illegals to Go Home" and has argued, "the citizenship question is necessary to collect the data for a redistricting of House seats that excludes aliens from the calculation."

Uthmeier had also solicited advice from Thomas Hofeller, a Republican redistricting specialist, who wrote in letters and memos that the question would create an electoral advantage for "Republicans and non-Hispanic whites. Hofeller has since died.

Former Kansas Secretary of State Kris Kobach, an immigration hard-liner who served on Trump's transition team, was also consulted. Documents revealed that Kobach urged Ross in a July 2017 email to add a citizenship question to address the "problem" that undocumented immigrants "are still counted for congressional apportionment purposes."

In a statement, a Commerce Department spokesperson refuted the committee's characterization of the testimony, claiming it mischaracterizes what Uthmeier said on multiple topics.

"In addition, Mr. Uthmeier, a former senior Department attorney, answered over 400 questions but declined, consistent with his ethical obligations, to discuss privileged communications. That was entirely appropriate," the spokesperson said.