.MINNEAPOLIS (AP) – Investigators with the National Transportation Safety Board continue to interview motorists who were on the Interstate 35W bridge when it collapsed more than two months ago and expect to conclude their onsite investigation in late November, the agency said Tuesday.

In the fourth update of its probe, the NTSB released little new information, saying 30 investigators were working on the investigation in Minneapolis and at the agency’s headquarters.

Investigators are interviewing motorists who have also been reviewing a diagram of the bridge that includes vehicles and equipment in their likely positions to refine estimates of how much weight was on the bridge when it fell Aug. 1.

Bridge components are being kept at the Bohemian Flats, an area near the site of the collapse, and will be shipped later to NTSB headquarters.

“Since the start of this accident investigation, we have made substantial progress in recovery, documentation, and data gathering,” NTSB Chairman Mark V. Rosenker said in a prepared statement. “However, several of our investigators are still on-scene and we are in the midst of a complete and thorough analysis of this information.”

The bridge collapse killed 13 people and injured more than 100. The NTSB has said it could take 12 to 18 months to determine a probable cause.