In case there was any mystery left, Julián Castro is running for president.

That's according to his twin brother, U.S. Rep. Joaquin Castro, D-San Antonio, who made the announcement during a joint interview Thursday night on CBS's "The Late Show with Stephen Colbert." As Colbert teased Julián Castro about the build-up to his all-but-certain 2020 presidential bid, Joaquin Castro interjected.

"I'll speak on his behalf here," Joaquin Castro said. "He's gonna run for president. How about that?"

The comment drew a grin and nod from Julián Castro, the former U.S. housing secretary and San Antonio mayor.

The joint interview came the day after he announced that he was forming a presidential exploratory committee and would make an announcement Jan. 12 in San Antonio. For months, however, he has hardly concealed his ambitions, releasing a book, visiting early voting states and even saying in recent weeks that he is "very likely" to run.