The House Judiciary Committee issued subpoenas to former Trump campaign manager Corey Lewandowski and former White House aide Rick Dearborn on Thursday as the impeachment investigation began to heat up.

Lewandowski and Dearborn were issued subpoenas to testify publicly before the committee on Sept. 17, regarding details about obstruction of justice by President Donald Trump. Both of the Trump aides have featured extensively in former special counsel Robert Mueller’s report.

The subpoenas came at a time when the committee is looking to speed up the investigation process and gather as many evidence as possible, so as to recommend the article of impeachment by the end of this year. The subpoenas have however been branded a waste of time and taxpayers money by a Republican Aide to House Judiciary Committee.

Citing Mueller’s reports, the Judiciary Committee pointed out two incidents involving Lewandowski as damning evidence of obstruction of Justice. The first was when Trump told Lewandowski to force then-Attorney General Jeff Sessions to limit the Mueller investigations or be fired. Lewandowski then asked Dearborn to deliver this message to Sessions.

In the second incident, the President again asked Lewandowski to force Sessions to limit Mueller’s investigations to future election interferences and not the current election meddling.

“Corey Lewandowski and Rick Dearborn were prominently featured in the special counsel’s description of President Trump’s efforts to obstruct justice by directing then-White House Counsel Don McGahn to fire the special counsel, and then by ordering him to lie about it,” Judiciary Committee Chairman Jerrold Nadler (D-N.Y.) said in a statement. “This will help the committee determine whether to recommend articles of impeachment against the president or other Article 1 remedies,” Nadler added.

Some Democrats have been pushing the committee to subpoena Lewandowski in order to avoid an executive privilege clash with the White House, as he is the only star witness in Mueller report who has no formal role in the White House. However, the White House officials have been in discussions to invoke the executive privilege to prevent Lewandowski from divulging any information.

Lewandowski had testified last year before the House Intelligence Committee but refrained from answering any questions about incidents that occurred post-2016 elections.

Lewandowski is considering to run for Senate from New Hampshire, but the GOP feels that recent developments against him might make it difficult for him to beat Sen. Jeanne Shaheen (D-N.H.).