But “a letter is not the same as a contract,” said Cassie Paben, the mayor’s deputy chief of staff for economic development.

Stothert said OPA later asked for more than $3 million. She referred questions about the details of the negotiation to HDR and OPA.

HDR didn’t respond to a request for additional details. Joan Squires, the arts group’s president, said OPA and HDR did have an agreement, but she wouldn’t elaborate on what might have fallen through. She referred a reporter to the group’s written statement.

In the statement, the group said it was “sad and disappointed” to learn that HDR wouldn’t build its headquarters downtown. The statement didn’t disclose any details about what, if anything, had soured the agreement.

“We had looked forward to welcoming the company and their employees to the neighborhood,” the statement said. “We appreciate HDR’s long-time support and partnership.”

HDR’s chairman and chief executive, George A. Little, said: “Regrettably, since the project did not develop as originally proposed, it is no longer in the best interests of our employee-owners and company to move downtown.”