Minneapolis ? A camera mounted onto a helicopter will snap close-up photos Sunday of the mangled north end of the Interstate Highway 35W bridge that crashed into the Mississippi River four days ago, as the search narrowed for the origin of the puzzling accident.

Federal investigators on Saturday ruled out any defects in the south end of the bridge that would have caused the steel-arched truss structure to break apart during Wednesday evening’s rush hour, spilling about 50 vehicles into the river 64 feet below.

A review also showed no significant seismic activity in the area, ruling out any type of Earth tremors as the cause of the collapse.

The investigation into what precipitated the catastrophic failure now moves elsewhere along the 1,900-foot bridge.

The below-roadway superstructure of the northern bridge section jutted upward as it crumbled – it’s now partly submerged below the water line, partly angled steeply skyward – preventing a hands-on visual inspection by the National Transportation Safety Board.

“If we begin to see anything there (based on photos taken from the helicopter), we would try to take a piece of it off and take it back to our laboratory for inspection. If not, our focus will then go toward the center section,” said safety board Chairman Mark Rosenker.

Meanwhile, the NTSB gave Minnesota officials permission to begin removing the eight-lane bridge deck and the vehicles that remained on it.

The center section and the super-structure below it will be excavated next. It will give investigators an opportunity to analyze those remaining sections, as well as clear the area for the eventual construction of a new bridge, officials said.

Rosenker said investigators have several theories about what sparked the accident, but he said it is too early to go public with them because new information may lead the probe into an entirely different direction.

“Overall we continue to make progress,” he said. “But these moves are very, very, very small measures of progress.”