The first fire was reported at 3:50 a.m. at an end apartment under construction in the Courtney Creek Apartments on Courtney Creek Boulevard near N.C. 55.

At 4:57 a.m., authorities were alerted to a fire that began in the back bedroom of a vacant house at 120 E. Cornwallis Road, just south of the city center.

The last fire clocked in at 6 a.m. at 2001 Great Bend Drive, off Geer Street in east Durham. The fire started in the upstairs bedroom of a home undergoing construction and spread through the floor into the garage below.

"All three fires are under investigation, but we can't say at this time if they are linked," said Castle, adding, "The fact that all three were vacant, and two were under construction, could indicate a link." Castle said it was not unusual that the fires "occurred right on top of each other." However, he said the timing was the "biggest hindrance" in the investigation, since there were no witnesses present. "Most people were asleep," Castle said. "We have investigators canvassing neighborhoods to find anything that would point to something suspicious."

The Courtney Creek fire started in the back of the end apartment and quickly destroyed it, according to Castle, who said a firewall saved neighboring units. The vacant house on Cornwallis sustained extensive water and smoke damage, Castle said. The investigator said monetary estimates of the damage were not available Friday. No residents or firefighters were injured because of the fires, and Castle said there were no complications in extinguishing them.

Castle said investigators found a pour pattern in the floor at the site of the third fire, indicating an accelerant was used. He said nothing similar was found during preliminary investigation of the other fires, adding that pour patterns are "usually almost impossible to find," due to the extent of fire damage.

Friday's incidents were the latest in a series of fire investigations with undetermined cause.

On Jan. 5, a fire demolished part of the Duke Inn, at 5139 Redwood Road. Durham County officials could not be reached for comment on the status of their investigation, but recent reports were unable to speculate about the fire's origins.

On Jan. 19, a propane tank explosion destroyed eight units at the Innesbrook Apartments off Fayetteville Road, leaving several families homeless. Inspector Lennis Harris said the cause of the fire was still under investigation, pending evidence reports from the State Bureau of Investigation.

"There's no timetable in fire investigations like this," Harris said. "You just have to do everything right, do everything carefully, to make sure everything's covered. This was a large fire -- that takes a little more time."