Housing and Urban Development Secretary Julian Castro violated a federal law against politicking on the job earlier this year when he fielded a journalist's questions about the presidential race and his own chances of being chosen as Hillary Clinton's running mate, a federal ethics watchdog agency has found.

The Office of Special Counsel determined that Castro ran afoul of the Hatch Act during his April 4, 2016, interview with Yahoo News anchor Katie Couric, even though he stressed that he was answering her political questions in his personal capacity.

"OSC concluded that Secretary Castro violated the Hatch Act by advocating for and against Presidential candidates," Special Counsel Carolyn Lerner wrote in a letter to President Barack Obama Monday. "Secretary Castro’s statements during the interview impermissibly mixed his personal political views with official agency business despite his efforts to clarify that some answers were being given in his personal capacity."

When Couric turned from official matters to politics in the interview, asking about his endorsement of Clinton in the presidential race, Castro said, "Now, taking off my HUD hat for a second and just speaking individually, it is very clear that Hillary Clinton is the most experienced, thoughtful, and prepared candidate for President that we have this year."

Castro also answered a question about his fears of a Trump presidency and about his own chances of being named as Clinton's vice presidential candidate. He rebuffed the latter question, saying: "What I am interested in, though, is trying to do a great job here at HUD and serving the people that we do serve, folks that are of modest means but who deserve our attention and our efforts. And so I don’t believe that is going to happen, but I am supportive of Secretary Clinton and I believe she is going to make a great president."

In a letter released by OSC, Castro said he thought he was complying with the law in how he conducted the interview, but now realizes he should not have allowed official and political topics to be mixed at all.

"In responding to a journalist's question about the 2016 election, I offered my opinion to the interviewer after making it clear that I was articulating my personal view and not an official position. At the time, I believed that this disclaimer was what was required by the Hatch Act. However, your analysis provides that it was not sufficient," Castro wrote.

"When an error is made - even an inadvertent one - the error should be acknowledged. Although it was not my intent, I made one here," Castro added.

Under the Hatch Act, rank-and-file government employees can be reprimanded, suspended, docked pay or even fired. However, sanctions against presidential appointees are up to the president, so reports on such individuals are sent directly to the president.

In her letter, Lerner proposed no penalty for Castro but said she hoped her offices findings would encourage other federal employees to abide by their obligations not to engage in political activities while engaged in official duties

"As the upcoming Presidential election approaches, this report offers an opportunity to deter violations by reminding federal employees of the Hatch Act’s restrictions," Lerner wrote.

A White House spokeswoman, Jen Friedman, made no mentioned Monday of any decision on punishment for Castro.

"Secretary Castro has acknowledged the inadvertent error he made in answering an interview in question, and has taken full responsibility for ensuring that such errors do not occur again," Friedman said. "As his letter to the Office of Special Counsel (OSC) indicates, the Secretary takes the Hatch Act very seriously and he has committed to taking steps to ensure understanding and compliance with the Hatch Act throughout the Department of Housing and Urban Development."

On occasion, Obama's Cabinet has faced questions about compliance with the Hatch Act.

In 2012, OSC found then-Health and Human Services Secretary Kathleen Sebelius violated the law in a speech to a gay rights organization when she advocated for Obama's re-election during a speech on Obamacare. She said said she went "off script" and made a "mistake."

Then-Labor Secretary Hilda Solis also came under investigation by both the OSC and, reportedly, the FBI over her alleged involvement in fundraising for Obama's campaign in 2012. The Hatch Act prohibits solicitation of political donations by federal employees, although they are permitted to speak at fundraisers in their private capacity.

The OSC probe was closed without action after Solis resigned from her post in 2013. The FBI investigation apparently ended without any charges being filed.

OSC pointed to what it considered wide-scale violations of the Hatch Act at the White House under President George W. Bush, including "a systematic misuse of federal resources." Former Bush aides disputed the findings and said many of the practices faulted by the watchdog agency had been commonplace for years under presidents of both parties.