A federal judge in mid-August had rejected a request by Schwebel, Goetz & Sieben to be allowed on site. At the time, a recovery operation was ongoing for people missing in the collapse.
The last victim was recovered Aug. 20. The law firm said its investigators were allowed into the site on Thursday following negotiations with the state.
The Aug. 1 collapse killed 13 people and injured about 100 others. A federal investigation of the cause is continuing.