(CNN) -- A fire erupted Friday at the headquarters of the Shanksville, Pennsylvania, memorial to United Airlines Flight 93, which crashed on September 11, 2001, officials said. "There is a potential for 9/11 memorabilia loss due to a fire," according to a statement from the National Park Service, but the extent of the damage is not known yet. NPS spokesman Mike Litterst said 10% of the archives and museum collection was stored in the damaged buildings, but much of it was kept in a fireproof safe. Among the items in the damaged buildings was a U.S. flag that flew over the U.S. Capitol on the day of the terrorist attack. The flag was given to the Flight 93 National Memorial last September 11. Its status was not known. Four buildings were damaged, the park service said. Heavy rain helped the firefighting efforts. Seven fire companies responded to the blaze, which started about 3 p.m., said Geraldine Budzina, a Somerset County dispatcher. No injuries were reported. "I think stuff has been lost" Ken Nacke, whose brother Louis was on Flight 93, said many 9/11 artifacts -- including notes and other mementos left at a temporary memorial at the crash site -- were being stored in the area where the fire burned. He said he had spoken Friday with the head of Families of Flight 93 shortly after the group president was contacted by the memorial's superintendent. "I think stuff has been lost," Nacke said. "I just don't know what has been lost. I know the layout of the buildings and I wouldn't be surprised. It's heartbreaking that we spent all this time to have this happen. All the blood sweat and tears that went into building this." Nacke helped raise funds for the Flight 93 National Memorial and advised in its planning. "At the temporary memorial, people would leave cards, T-shirts and handmade stuff that was very comforting to us," he said. "That's why it's heartbreaking. I hope none of this stuff is lost." Cause of fire is unknown . Clouds of dark smoke could be seen billowing into the blue sky over the grounds Friday afternoon on a live webcam. "We are deeply saddened to learn that a fire occurred at the Flight 93 National Memorial headquarters," Gordon Felt, president of Families of Flight 93 said in a statement. "We understand that no one was injured, and we are grateful for that." "Neither the memorial proper nor the new visitor center currently under construction were affected, as the headquarters is located approximately two miles from those sites," Litterst said. Initial reports were of extensive damage to the complex, Litterst said. All employees and volunteers were safely evacuated. The cause of the blaze is under investigation, he said. 40 people died in the plane crash . The memorial park is dedicated to the 40 passengers and crew who died when Flight 93 crashed outside the town in southwestern Pennsylvania. The plane went down, killing all on board, as passengers fought back against the hijackers, according to investigations. The memorial, still incomplete, includes a visitor center with traditional and interactive exhibits, public programs and information about the history of Flight 93. United Airlines Flight 93 was traveling from Newark, New Jersey, to San Francisco when hijackers took over the plane, according to the 9/11 Commission. Investigators said the terrorists were most likely trying to turn the airplane toward Washington to hit a major political target. The 2,200-acre memorial park is managed by the National Park Service. The second phase of construction is to be completed by 2015. Since 2001, more than 1 million people from around the world have visited the crash site. What you need to know about the Flight 93 National Memorial . CNN's Aaron Cooper, Deanna Hackney and Laura Bernardini contributed to this report.
NEW: Some 911 artifacts may have been damaged, park service says .
Fire breaks out at headquarters of memorial to United Airlines Flight 93 .
Headquarters complex is about two miles from the memorial in Pa.
Flight 93 crashed on September 11, 2001, after hijackers took over the plane .
(CNN) -- A fire erupted Friday at the headquarters of the Shanksville, Pennsylvania, memorial to United Airlines Flight 93, which crashed on September 11, 2001, officials said. "There is a potential for 9/11 memorabilia loss due to a fire," according to a statement from the National Park Service, but the extent of the damage is not known yet. NPS spokesman Mike Litterst said 10% of the archives and museum collection was stored in the damaged buildings, but much of it was kept in a fireproof safe. Among the items in the damaged buildings was a U.S. flag that flew over the U.S. Capitol on the day of the terrorist attack. The flag was given to the Flight 93 National Memorial last September 11. Its status was not known. Four buildings were damaged, the park service said. Heavy rain helped the firefighting efforts. Seven fire companies responded to the blaze, which started about 3 p.m., said Geraldine Budzina, a Somerset County dispatcher. No injuries were reported. "I think stuff has been lost" Ken Nacke, whose brother Louis was on Flight 93, said many 9/11 artifacts -- including notes and other mementos left at a temporary memorial at the crash site -- were being stored in the area where the fire burned. He said he had spoken Friday with the head of Families of Flight 93 shortly after the group president was contacted by the memorial's superintendent. "I think stuff has been lost," Nacke said. "I just don't know what has been lost. I know the layout of the buildings and I wouldn't be surprised. It's heartbreaking that we spent all this time to have this happen. All the blood sweat and tears that went into building this." Nacke helped raise funds for the Flight 93 National Memorial and advised in its planning. "At the temporary memorial, people would leave cards, T-shirts and handmade stuff that was very comforting to us," he said. "That's why it's heartbreaking. I hope none of this stuff is lost." Cause of fire is unknown . Clouds of dark smoke could be seen billowing into the blue sky over the grounds Friday afternoon on a live webcam. "We are deeply saddened to learn that a fire occurred at the Flight 93 National Memorial headquarters," Gordon Felt, president of Families of Flight 93 said in a statement. "We understand that no one was injured, and we are grateful for that." "Neither the memorial proper nor the new visitor center currently under construction were affected, as the headquarters is located approximately two miles from those sites," Litterst said. Initial reports were of extensive damage to the complex, Litterst said. All employees and volunteers were safely evacuated. The cause of the blaze is under investigation, he said. 40 people died in the plane crash . The memorial park is dedicated to the 40 passengers and crew who died when Flight 93 crashed outside the town in southwestern Pennsylvania. The plane went down, killing all on board, as passengers fought back against the hijackers, according to investigations. The memorial, still incomplete, includes a visitor center with traditional and interactive exhibits, public programs and information about the history of Flight 93. United Airlines Flight 93 was traveling from Newark, New Jersey, to San Francisco when hijackers took over the plane, according to the 9/11 Commission. Investigators said the terrorists were most likely trying to turn the airplane toward Washington to hit a major political target. The 2,200-acre memorial park is managed by the National Park Service. The second phase of construction is to be completed by 2015. Since 2001, more than 1 million people from around the world have visited the crash site. What you need to know about the Flight 93 National Memorial . CNN's Aaron Cooper, Deanna Hackney and Laura Bernardini contributed to this report.
of the Shanksville, Pennsylvania, memorial to United Airlines Flight 93, which crashed on September 11, 2001, officials said. "There is a park service said. Heavy rain helped the Louis was on Flight 93, said many 9/11 artifacts approximately two miles from those sites," hijackers took over the plane, according to the