It will take the D.C. Department of Transportation three more days to fill a gaping sinkhole downtown, in what one official calls one of the most complex street repairs in the past decade.

The sinkhole, located at 14th and F streets, is expected to be filled in by Friday, according to DC Water General Manager George Hawkins. Officials originally believed the hole was caused by a piece of concrete breaking a sewer line. But Hawkins said a preliminary investigation reveals a manhole blocked the sewer drain and caused rainwater to flow into the ground under the road. The excess water eroded the dirt underneath the street, which caused a piece of concrete to fall onto the sewer line.

"This is very, very unusual," Hawkins said. "This is the most complex street repair that I've seen since working here, at least in the past decade."

Hawkins said he did not know how the manhole was installed in the road, and that it wasn't one of DC Water's. It is still unclear to officials whether the manhole built into the ground initially penetrated the sewer line, or if the manhole slowly sank into it and cracked it.

Because the sewer line is about 18 feet into the ground, crews had to slowly work their way down to gain access to it and assess the damage. Repairing the line will cost about $2 million.

The damage has caused the northbound lanes on 14th Street and both lanes on F Street between 13th and 15th streets to be closed since May 21st.

City officials believe the water erosion that caused the sinkhole has been going on for at least a few years.

D.C. Mayor Vincent Gray said he and other city officials had no idea there were any problems with the sewage line that caused the sinkhole.

"Certainly, there's no way we would have ignored something that serious," he said. "We got on it as soon as we knew that it was a problem."