Cook tweeted a brief apology Friday, saying he didn't want any of his new material to hit the airwaves yet. That hasn't pacified critics, including several who asked on Twitter why Cook chose to do a new set at a charity event.
Cook's publicist didn't immediately respond to requests for comment.
Thursday night's Boston Strong Concert featured acts including Aerosmith, James Taylor, Jimmy Buffett, New Kids on the Block and Boyz II Men. The show raised money for The One Fund Boston, a compensation fund established by Gov. Deval Patrick and Mayor Thomas Menino to help bombing victims.
A representative of concert organizers sought to defend Cook, a Massachusetts native.
"Dane donated both his time and incredible talent to yesterday's event and was most certainly an integral part of making the evening an extraordinary success benefiting The One Fund," Jay Anderson of Live Nation said in brief emailed remarks. He declined to comment further, referring all questions to Cook's publicist.
Authorities allege that two brothers, ethnic Chechens from Russia, set off two pressure cooker bombs packed with shrapnel near the marathon's finish line on April 15, killing three people and injuring more than 260 others. Their mother has insisted that they are innocent.
Dzhokhar Tsarnaev was found hiding in a boat in a backyard in suburban Watertown. His older brother, Tamerlan Tsarnaev, died after his younger sibling ran over him as police officers struggled to subdue him following a police chase in which Tamerlan was shot.