A sinkhole yawned open last week near the Mall in one of the city’s major traffic arteries, and by week’s end, despite diligent investigation and lots of digging, the cause of the impediment remained a mystery.

The hole, about two feet wide and four feet long, was reported Tuesday in the northbound lanes of 14th Street NW between Constitution and Pennsylvania avenues. It was originally believed that a leak from a broken sewer pipe might have eroded soil under the street.

With part of the heavily used street closed, crews from the city’s water utility dug a trench four feet wide by 27 feet long to permit them to find and fix the suspected breach. But D.C. Water spokesman Emanuel D. Briggs said the workers found that there “is no water or sewer infrastructure located directly beneath the site of the sinkhole.”

Moreover, he said, the pipe that they did inspect was in “proper working condition.”

What did cause the collapse, he said, “is the million-dollar question.”

He said efforts to find the answer are still underway.

Meanwhile, after the area of the hole was closed to traffic for much of last week, temporary repairs were made to permit its use over the weekend.

After time for concrete to cure, the area will probably close again Monday to permit complete restoration, Briggs said.

As far as he knew, he said, there is no link between the current problem and a notorious sinkhole that occurred last year at 14th and F streets NW.

That hole, a few blocks north of the much smaller one found last week, took days to repair and affected traffic severely.