A fire last weekend at the construction site of a future mosque in Murfreesboro, Tennessee, has been determined to be arson, a Bureau of Alcohol, Tobacco, Firearms and Explosives spokesman said Friday.

Lab reports indicate that accelerants were used to start and spread the fire, which destroyed an earth mover and damaged three other vehicles at the future site of the Islamic Center of Murfreesboro, ATF spokesman Eric Kehn said.

There are no suspects in the arson, which occurred early Saturday morning, Kehn said. The investigation is ongoing and the ATF and FBI are offering a $20,000 reward for information leading to the arrest of a suspect or suspects, authorities said at a Friday press conference at the construction site.

"Somebody here in Rutherford County knows what happened here," said Keith Moses, an FBI assistant special agent based in Nashville, at the press conference, which featured representatives from the Islamic center.

"Whether or not we have a civil rights hate crime will be determined once we have a suspect or suspects and a motive," Moses said.

The FBI, ATF, and Rutherford County Sheriff's Office are conducting an ongoing investigation. Federal authorities and members of the Islamic center had suspected that the fire was intentionally set.

"We were expecting to hear it but in the back of our minds we were hoping for the best, that it was some kind of electrical fire," Camie Ayash, a spokeswoman for the Islamic center, told CNN on Friday. "It ingrained into our heads that this is definitely arson and that somebody did intentionally go out and do this."

The Islamic center's board has decided to hire private security for the site, Ayash said, after the contractor for the project suggested it. She expects a private security firm to start monitoring the site after hours beginning next week.

The Rutherford County Sheriff's Office has stepped up patrols of the site since the fire, driving by about every 30 minutes, she said.

The blaze has "really raised the fear factor" among area Muslims, Ayash told CNN earlier this week.

"We see the different type of fear with our children," she told CNN's "American Morning."

"It is very hard to explain to children what is going on. It is hard to explain to the little kids when they ask you, 'Mommy, are these people for us or against us?' "

A candlelight vigil, organized by Middle Tennesseans for Religious Freedom in response to the fire, drew about 100 people to the Rutherford County Courthouse on Monday night.

The Islamic Center of Murfreesboro has existed in the Murfreesboro area for more than a decade, according to its website, and currently meets about a mile from the site of the future mosque.

The congregation purchased a 15-acre plot in 2009 and announced plans for a center that will include a mosque, educational facilities, a gym, cemetery and various recreational areas, including tracks, pavilions and a playground.

The project has provoked controversy in Murfreesboro, about 35 miles southeast of Nashville, and statewide.

In July, several hundred opponents of the mosque staged a march against the project. Some objected to Islam itself, carrying signs like "MOSQUE LEADERS SUPPORT KILLING CONVERTS," while others opposed the project for environmental reasons.

Last month, Tennessee Lt. Gov. Ron Ramsey publicly criticized the project. "You could even argue whether being a Muslim is actually a religion, or is it a nationality, way of life, a cult, whatever you want to call it," Ramsey, then a candidate for Tennessee governor, said at a rally.

Ramsey placed third in Tennessee's Republican primaries last month.