Some were deliberately lit. Others were caused by an electrical fault or lightning. But regardless of the origins, a spike in fires at black churches is fraying nerves at a time of heightened racial tensions across the United States.

As federal investigators search for clues, the fires highlight what appears to be a little-known feature of American religion: Hundreds of churches go up in flames every year for varied reasons, often having nothing to do with race or violence.

Still, recent images of black churches ravaged by fire are alarming after last month's racially charged massacre at a black church in South Carolina and in regions in the South with long histories of violence against black churches.

In Greeleyville, South Carolina, just 65 miles (105 km) north of the church where self-proclaimed white supremacist Dylann Roof allegedly killed nine black parishioners, investigators took samples from the charred rubble of the Mount Zion African Methodist Episcopal Church.

They said they would assess whether an accelerant had been used to deliberately fan Tuesday night's blaze, which unsettled many black activists and members of a church, which the Ku Klux Klan burned to the ground 20 years ago.

It was too early to determine what caused the latest fire, said Thom Berry, a spokesman for the South Carolina Law Enforcement Division. "We don't want to speculate at this stage," he added.

In total, seven predominantly black churches have caught fire since the South Carolina shooting. Three are being investigated as arson, according to Reuters interviews with local authorities and church leaders.

"We're in a state of shock," said Brandon Reeves, a member of God's Power Church of Christ in Macon,Georgia. The church, founded by his great-grandmother Lillie Powell, was destroyed by fire on June 23. "With all these other churches in flames, I can't help but think it might have been a hate crime."

Authorities have not classified that fire, or any others, as a hate crime. But it appears to be arson, saidMacon-Bibb County spokesman Chris Floore.

In Knoxville, Tennessee, bags of dirt and bales of hay were set on fire outside the doors of the College Hill Seventh Day Adventist church on June 21, and a van was torched. "We're treating it as arson, but this was more vandalism than anything," said Captain DJ Corcoran of the Knoxville Fire Department.

"WHO DID THIS?"

In Washington, Bureau of Alcohol, Tobacco, Firearms and Explosives spokeswoman Ginger Colbrun said preliminary investigations suggested the fires were not racially linked.

That does little to allay those affected by the fires, such as Rhonda Kinsey, co-pastor at Briar CreekBaptist Church in Charlotte, North Carolina, where a June 24 fire caused more than $250,000 in damage and is being investigated as arson.

"We're just trying to get to the bottom of why or who did this," she said.

Twenty-five years ago her church was an all-white evangelical ministry. Today, she describes it as mostly African-American with some white and Hispanic members.

The NAACP urged black churches to "remain vigilant" and take "necessary precautions."

In Elyria, a city of 54,000 people in Ohio, Pastor Derrell Deer said an electrical fault sparked a raging fire at his mostly black College Heights Baptist Church on Saturday morning.

"It didn't appear to be anything that was intentional,' he said.

Such was the case in Tallahassee, Florida, where the Greater Miracle Temple Apostolic Holiness Church was destroyed. A tree limb fell on an electrical service line connected to the church, sparking Friday's pre-dawn fire, authorities said.

Arson was also ruled out in a June 26 fire at the Glover Grove Missionary Baptist Church in Warrenville,South Carolina, said Captain Eric Abdullah of the Aiken County Sheriff's Office.

Not all the churches that went up in flames over the past two weeks were African American. On June 23, lightening struck the roof of the mostly white Fruitland Presbyterian Church in Memphis, Tennessee.

"At first I thought it hit my house, but it struck the church across the street," said Hazel Moss, 79, a neighbor who witnessed the fire. "I got up, noticed my window was glowing red, and thought 'what in the world is going on', and then I saw the fire."

According to the National Fire Protection Association, officials responded to 1,660 fires at religious and funeral properties in 2011, the most recent year for which data is available. About 16 percent of those were intentionally set, for an average of about five arsons a week, it said.

"Most of these fires are small," said Marty Ahrens, manager of fire analysis services at the association. "Churches are often historic structures, depending on the part of the country you're in. They may or may not have been built to current code, and trying to maintain them can be a challenge."