The Senate Judiciary Committee on Tuesday suddenly postponed a hearing that would have centered on the so-called Trump dossier and several of the people involved in the June 2016 Trump Tower meeting that is currently in the news.

Iowa Sen. Chuck Grassley, the chairman of the committee, made the decision to postpone the hearing, which was scheduled for Wednesday.

It is not entirely clear why the hearing is being postponed, though one of the main witnesses, Glenn Simpson, the opposition researcher behind the dossier, was reportedly planning to skip the event.

A spokesperson for the committee said that the hearing is being postponed as Grassley and California Sen. Dianne Feinstein work to schedule additional witnesses and obtain relevant documents.

The committee did not say whether Simpson will be subpoenaed.

Grassley has pressed Simpson and his firm, Fusion GPS, about its involvement in the dossier, which was prepared by former British spy Christopher Steele. Fusion GPS hired Steele after being hired by a Democratic ally of Hillary Clinton’s to investigate Trump’s ties to Russia.

Wednesday’s two-part hearing was to focus on failures in the Foreign Agents Registration Act, the law which requires lobbyists and consultants working for foreign governments to disclose their activities.

Simpson, a former Wall Street Journal reporter, was invited to take part in a second panel along with Bill Browder, a London-based financier who helped push through the Magnitsky Act, a sanctions bill passed in 2012.

Last year, well before Simpson’s involvement in the dossier was known, Browder filed a complaint with the Justice Department alleging that the opposition researcher failed to properly file under FARA.

Fusion GPS had been hired to conduct opposition research against Browder as part of a lobbying campaign against the Magnitsky Act, which is named after Browder’s attorney, Sergei Magnitsky.

The Russian government vehemently opposes the bill, which blacklists Russian businessmen involved in human rights abuses.

Browder’s work on the Magnitsky Act, and his FARA complaint, have received renewed attention following the publication of the Steele dossier as well as the June 2016 Trump Tower meeting.

Three of the attendees in that meeting were named in Browder’s complaint.

They are Natalia Veselnitskaya, a Russian lawyer, Rinat Akhmetshin, a Russian-American lobbyist and former Soviet military officer, and Anatolia Samochornov, a translator who worked as a State Department contractor until September.

The three Russians were affiliated with a non-profit group called the Human Rights Accountability Global Initiative Foundation (HRAGIF).

The group claimed to be an advocate for U.S. adoptions of Russian children, which were blocked by Russian president Vladimir Putin as retaliation for the Magnitsky Act. But in reality, the group and its trio of operatives, were interested in rolling back Magnitsky.

Browder told The Daily Caller in January, just after the dossier was published by BuzzFeed, that he questioned the veracity of the document because of Simpson’s involvement.

The overlap between the dossier and the anti-Magnitsky push has led some Trump supporters to question whether the Trump Tower meeting was an attempt to set up or compromise the Trump campaign.

Donald Trump Jr. agreed to the meeting after being told by an acquaintance that he would be provided with derogatory information about Hillary Clinton. Trump Jr. was told that the information was being provided with the backing of the Russian government and by a “Russian government attorney.”

A lawyer for Fusion GPS has said that the firm was not aware of the meeting before it took place.

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