First responders quickly determined there were no survivors of the Flight 93 crash, and the Pennsylvania State Police secured the site as a federal crime scene. The September 11 attack led to the largest investigation in FBI history, with hundreds of investigators combing the crash site for evidence. Agents verified the terrorists' identities as members of al-Qaeda, and investigators soon realized that among the four crash sites, the Shanksville location would likely yield the most evidence because Flight 93 crashed in an open field, where parts of the plane, personal effects, and human remains could be recovered more easily than at the World Trade Center and Pentagon. The FBI excavated the impact crater to find the black boxes and searched the nearby grove of hemlock trees and surrounding field for clues, recovering several knives, documents in Arabic detailing the attack plan, passports, and a bank card belonging to a terrorist. On September 24, 2001, the FBI concluded its field investigation at the Flight 93 crash site. Although investigators recovered only a small percentage of human remains, scientists working with the coroner identified all onboard through DNA analysis. By early October, the excavated crater was backfilled, scorched trees were removed, and the site was planted with grass and wildflowers.