Washington(CNN) A House Democrat has opened an investigation into Secretary of State Mike Pompeo's controversial decision to address the Republican National Convention.
Rep. Joaquin Castro, the chairman of the House Foreign Affairs Oversight and Investigations Subcommittee, said in a statement Tuesday that the "American people deserve a full investigation."
The decision to speak to the political convention in prerecorded remarks from Jerusalem breaks with longstanding precedent of sitting secretaries of state avoiding partisan politics, particularly while abroad, and appears to violate guidance on political activities that Pompeo himself emphasized in a cable to diplomats just last month. It has rankled current and past State Department officials.
In a letter to Deputy Secretary of State Stephen Biegun on Tuesday, Castro noted, "It is highly unusual, and likely unprecedented, for a sitting Secretary of State to speak at a partisan convention for either of the political parties. It appears that it may also be illegal."
The Texas Democrat cited the Hatch Act and State Department guidance -- including a December memo from the State Department's Office of the Legal Adviser that "Senate-confirmed Presidential appointees may not even attend a political party convention or convention-related event."
"In furtherance of the constitutionally mandated obligation to conduct oversight of the State Department, as well as to determine whether and legislative action is necessary, to address this matter, the Subcommittee on Oversight and Investigations intends to examine this issue carefully," Castro wrote.
Pompeo recorded his speech, which is slated to air at the convention Tuesday, while in Jerusalem on an official State Department trip to the Middle East and North Africa.
A State Department spokesperson and another source familiar with the situation defended the move Sunday by saying Pompeo would deliver the remarks in his personal capacity and that no taxpayer funds would be used. The source said the speech was cleared through Pompeo's personal lawyers, State Department and White House lawyers, and RNC lawyers.
Castro called for the second-highest ranking diplomat to arrange for a briefing and provide written answers by September 1 to a number of questions related to that legal authorization, the use of State Department resources and staff, the planning and itinerary of Pompeo's official trip to the region, and whether Biegun will be making referrals to the department's inspector general or the Office of the Special Counsel, which investigates Hatch Act violations.
In addition, Castro asked Biegun to provide by September 10 "(a)ny and all records prepared by or sent to or from the Office of the Legal Adviser on or after December 3, 2019 regarding restrictions on political activities by Secretary Pompeo or Presidential Appointees or Political Appointees; and (a) list of expenditures from the trip during which Secretary Pompeo recorded his remarks to the RNC, including receipts demonstrating that any expenses incurred by the State Department were reimbursed by the RNC, Trump's campaign, or other outside sponsors."
CNN has reached out to the State Department for comment on Castro's investigation.
On Tuesday, Pompeo tweeted from his personal account, "President Trump has ensured the safety of America — and SECURED our many FREEDOMS, which is the cornerstone of this great nation."
"I look forward to sharing with you how the President has delivered on that mission TONIGHT!" he wrote.
Just a month earlier, he warned staff in a diplomatic cable to "not improperly engage the Department of State in the political process," and stressed departmental guidance on political activities.
In the July cable, the top US diplomat noted that "presidential and political appointees and career SES (Senior Executive Service) are subject to significant restrictions on their political activity; they may not engage in any partisan political activity in concert with a partisan campaign, political party, or partisan political group, even on personal time and outside of the federal workplace."
"The Department works to advance the national interest abroad on behalf of all Americans in a non-partisan fashion. It is important to remember that in order to avoid any confusion or misperception in this regard, the Department's longstanding policy is that U.S. citizen employees and family members may not engage in partisan political activity while posted or on (temporary duty travel) abroad, even on personal time," he said.
And in an email sent in February, Biegun told employees he would "be sitting on the sidelines of the political process this year" and urged them to review guidance on political activities to ensure they are "above reproach."
Past secretaries of state from both sides of the aisle, including John Kerry, Hillary Clinton, Colin Powell and Condoleezza Rice, did not address their party's conventions while serving in the role.
Nick Merrill, a spokesman for Clinton, told CNN that "she didn't play a role because that's not what SecStates do. When you're serving in government, your job is to serve the American people, not your party's delegates."
"Like Pompeo, she was thinking of running for President, but her job as the country's chief diplomat, and her task of protecting our national security, came first, always," he said.
Pompeo's speech to the GOP convention is not the first time he has waded into the domestic political fray while serving as the top US diplomat.
Earlier this year, Pompeo -- who is thought to harbor 2024 White House ambitions -- spoke at the Conservative Political Action Conference and the conservative Family Leadership Summit in Iowa. His numerous trips to America's heartland amid speculation of a Kansas Senate bid drew scrutiny from congressional Democrats, though he ultimately decided not to run.
And he has delved into domestic culture wars, recently seeming to fan the flames of division stoked by President Donald Trump by warning that "the very core of what it means to be an American, indeed the American way of life itself, is under attack" amid nationwide protests for racial justice and against police brutality.