Castro is a top surrogate for Clinton's presidential campaign and -- while not expected to be the choice -- is often mentioned on lists of vice presidential prospects.
He violated the Hatch Act, which prohibits federal workers acting in their official capacity from attempting to influence elections. In the Couric interview, he appeared with the official HUD seal, was introduced as the secretary and spent the first seven minutes focused on HUD programs before Couric turned to Clinton.
The Office of Special Counsel wrote
: "Although he stated during the interview that he was 'taking off my HUD hat for a second and just speaking individually,' to indicate he was answering questions in his personal capacity, that disclaimer could not negate the fact that he was appearing in his official capacity for the rest of the interview, as the official HUD seal remained behind him and his political comments were bracketed by statements concerning official HUD policies and programs."
"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," Castro wrote. "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."
"Thank you for bringing this matter to my attention," he 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."
Obama's administration is barring cabinet secretaries from speaking at the upcoming Democratic National Convention in Philadelphia.
"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," said White House spokeswoman Jennifer Friedman. "As his letter to the Office of Special Counsel 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."