Rep. Lamar Smith, chairman of the House Science, Space, and Technology Committee, said Thursday that his panel planned to hold the firm that managed Hillary Clinton's email server in contempt of Congress, the first time he said his committee has ever taken such an "unprecedented" step to enforce a subpoena.

Smith said Clinton's legal team had sent emails to Platte River Networks, the company in question, and two other firms, and wanted the companies to withhold documents from his committees. He charged Platte River resisted congressional attempts to investigate whether Clinton purposefully deleted classified or other work-related emails from her server.

"These emails reveal not just a smoking gun, but who pulled the trigger," Smith said, describing the "delay tactics" Clinton's lawyer had employed to block the congressional probe.

The Republican chairman said his committee planned to vote on a contempt resolution for Platte River before the end of the year. If it passes, the contempt resolution will move to the House floor for a full vote.

"I see our investigation as completely separate from presidential politics," Smith said. But he argued Clinton's efforts to suppress responses from the firms that handled her emails suggests they are concealing evidence.

"They should not be surprised by people concluding that they have something to hide if they are not going to respond," Smith said.

Datto, Inc., which provided cloud-based storage for the emails, and SECNAP, which provided threat monitoring for the server, both cooperated with the committee by providing receipts and emails to investigators whenever possible.

Platte River stood out as the most uncooperative of the firms involved. It had also played the most direct role in mishandling and, ultimately, deleting classified material.

Republican aides on the Science Committee said correspondence with the three technology companies indicates the Clintons, through their personal attorney David Kendall, had instructed Platte River to flout congressional subpoenas.

Paul Combetta, an IT specialist at Platte River, received an immunity deal from the Justice Department in exchange for his cooperation with FBI agents. Combetta admitted, after securing the deal, that he used a digital deletion tool called BleachBit to destroy thousands of Clinton's emails beyond forensic recovery.

The Platte River employee also refused to cooperate with the House Oversight Committee last month in a separate investigation related to the email controversy, this one into the Justice Department's handling of the investigation.

Combetta invoked his Fifth Amendment rights, declined to answer questions and walked out of the hearing room. Another IT aide employed directly by the Clintons, Bryan Pagliano, did not show up to the Oversight Hearing or respond to his summons to appear.

Rep. Jason Chaffetz, chairman of the Oversight Committee, threatened to hold Pagliano in contempt.

Smith's probe of the cybersecurity measures applied to Clinton's server is one of several congressional investigations into her emails that have stalled when confronted with uncooperative witnesses or federal agencies.

Critics have accused the Justice Department in particular, and the administration in general, of slow-walking their responses to Congress in order to delay the probes past Election Day.