RALEIGH, N.C. (AP) — A Fort Bragg soldier canceled a concert with a secular theme and no ties to religion, saying Thursday that base leadership failed to honor promises to support his show as it had a Christian rally last fall.

Now, an advocacy group that watches for signs of religious bias in the military says it plans to file a lawsuit over the cancellation, arguing that Fort Bragg leaders have shown clear favoritism toward evangelical Christians at the expense of other groups.

"It's heartbreaking," said Sgt. Justin Griffith, who announced the cancellation in a Thursday letter to Col. Stephen Sicinski, the garrison commander at Bragg. "I'm personally invested in this, both in money and time. And now I feel like I've strung people along."

Calls to Bragg were not immediately returned Thursday evening.

Mikey Weinstein, president of the Military Religious Freedom Foundation and a planned speaker for the event, said his group plans to file the lawsuit.

"We're not going to negotiate our constitutional rights," he said. "We're going to federal court."

Fort Bragg is one of the military's largest bases. After a merger this week with what was Pope Air Force Base, the post now has about 58,000 uniformed military personnel and roughly 13,000 civilian workers.

Plans for the Rock Beyond Belief concert, which was planned to include famed British atheist Richard Dawkins as its keynote speaker, began last fall, after an event on the post called Rock the Fort.

That event, sponsored in part by the Billy Graham Evangelistic Association, was criticized by groups like Americans United for the Separation of Church and State as an attempt to evangelize not just soldiers, but civilians, who could also attend the show.

Military leaders said at the time they would support similar events by non-Christian groups, a promise reiterated in a letter last month from the Office of the Staff Judge Advocate at Bragg to the Freedom From Religion Foundation.

"Fort Bragg continues to be willing to provide the same level of support to comparable events proposed by non-federal entities," wrote Lt. Col. Nelson Van Eck Jr.

Griffith and other organizers say that changed abruptly this week, when they were told that they couldn't have the large outdoor gathering with games and activities they had planned. Instead, they were told the event could take place at one of two indoor theaters, with the larger one accommodating about 700 people.

In his letter to Sicinski, Griffith also says the base declined to cover any of the costs for Rock Beyond Belief, while it paid $54,500 toward the Christian event.

Griffith also says he was told that any advertisements for the secular event would have to carry disclaimers that the concert wasn't endorsed by Bragg, while the Christian event was explicitly endorsed by the post.

A military chaplain, for example, had sent out letters on Army stationary to area Christian pastors asking for their assistance in running the Christian concert.

Because of the disclaimer, the financial support and the venue change, Griffith said, the concert he planned wasn't able to go forward, which has left him disappointed and frustrated.

"This happened at the last minute," he said. "I just don't know how to pursue this further without litigation."