A federal grand jury has indicted a second Boston City Hall official in the alleged extortion of a music festival, the U.S. Attorney's Office announced Wednesday, furthering a federal probe that is rocking Mayor Martin Walsh's administration. Tim Sullivan, Boston's chief of staff for intergovernmental relations and senior advisor for external relations, was arrested Wednesday morning, according to federal prosecutors.

Sullivan, 36, lives in Dorchester, according to federal prosecutors. Ken Brissette, the 52-year-old tourism chief for the city, was arrested in May and charged in connection with his alleged involvement in the scheme.

The new indictment charges both Sullivan, a former AFL-CIO official, and Brissette with extortion and conspiracy to commit extortion on a Boston music festival widely acknowledged to be Boston Calling. Federal prosecutors say Sullivan and Brissette met with the festival organizers in September 2014, just three days before Boston Calling was set to begin on City Hall Plaza with musical acts such as Neutral Milk Hotel, The Roots and The Replacements, according to the fest's promotional materials.

At the meeting, they allegedly told the organizers half its labor force needed to be members of the International Alliance of Theatrical Stage Employees (IATSE) Local 11. That afternoon, the festival hired eight IATSE laborers and one foreman "as a result of the demands," and the city soon after issued the festival its necessary permits, the indictment states.

"If these allegations are true, simply really disheartening and concerns me. I mean I love being the mayor of the city of Boston. I hold everyone to a high standard of integrity," Walsh told reporters Wednesday morning at a coffee meet-and-greet in the North End's Christopher Columbus Park.

He said, "We don't encourage people to use union labor. There's a process in place. We have 800 events a year in the city of Boston and the topic of union labor should never come up, and to my knowledge it hasn't."

The mayor noted he has brought in outside counsel to look at the allegations and initiated ethics training for all department heads and cabinet members. He said both Sullivan and Brissette are on paid administrative leave until the court proceeding is complete.