“Please be aware that the Committees will resort to compulsory process to obtain the required testimony,” the lawmakers warned, asking that staffers from the three agencies work with committee staff to begin scheduling interviews as soon as possible, and no later than July 13.

Hunter Biden, President Joe Biden’s son, made a plea deal with federal prosecutors earlier this month to settle an investigation into his failure to pay about $1 million in federal income taxes and his purchase of a handgun in 2018 while being a user of illegal drugs, a felony. Under the agreement, Hunter Biden will plead guilty to a pair of misdemeanor tax charges and the gun charges will be dropped if he completes two years of probation.

Days later, Smith released testimony from two IRS whistleblowers who alleged a wide range of interference in the tax probe.

The unnamed IRS investigator who initially opened the investigation and his supervisor, Gary Shapley, accused the Justice Department and the Delaware U.S. Attorney’s office of slow-walking the case, blocking enforcement actions by the IRS and tipping off Hunter Biden’s attorneys about actions related to the investigation.

Chris Clark, an attorney for Hunter Biden, has denied there was any preferential treatment.

In their letters, Smith, Jordan and Comer named 11 employees of the Justice Department and two from the IRS whom they want to interview. The lawmakers did not name any specific Secret Service personnel, asking Cheatle to make available “all Secret Service employees” aware of an alleged “tip-off” from the FBI that IRS criminal investigators planned to interview Hunter Biden.

The lawmakers said they “anticipate” seeking testimony from other IRS, Justice and Secret Service employees.

A Justice Department spokesperson acknowledged receipt of the letter to Garland but said the department would have no further comment.

POLITICO has also contacted the IRS and Secret Service for comment.