The United Airlines Boeing 767 that came in to land at Newark Liberty International Airport on Sunday struck a semitrailer truck and a light pole on the New Jersey Turnpike, narrowly avoiding a disaster as it descended, the National Transportation Safety Board said. The NTSB later reclassified the incident as an accident, with the agency saying the plane’s damage extent drove the change in classification. The incident underscored how low planes can fly on approaches near the airport and how little margin exists when aircraft are operating close to roadways.

The truck driver, who was delivering bakery goods, was treated at a hospital for minor injuries, while the flight—flying in from Venice, Italy with 231 people aboard—was able to land safely and with no injuries reported to anyone on the plane. The bakery company that owned the delivery, H&S Family of Bakeries, said it was “relieved that everyone is safe, as that is our top priority,” and said it was cooperating with investigators.

The NTSB said Monday afternoon that it has reclassified the incident as an accident but did not provide details at the time. The agency said investigators arrived on scene on Monday to interview the flight crew and begin determining how and why the incident occurred, and it said it does not plan any news conferences. The NTSB said it expects to publish a preliminary report roughly a month from now.

An air traffic control audio posting described a discussion after the incident suggesting the aircraft had been damaged. In that audio, a controller said, “They felt something over the threshold and there’s a hole in the side of the airplane.” The cluster reporting also said air traffic control audio suggests the incident may have created a hole in the side of the plane, but that the airline and the NTSB had not confirmed that as of the initial report.

Witnesses on the turnpike described the plane’s height and the resulting impact. Patrick Oyulu, of Edison, New Jersey, told The Associated Press that “The plane seemed to come in almost directly over the highway,” adding that the truck appeared to swerve sharply and that smoke was visible after contact. Oyulu said he “never expected a plane that low” and that he saw the undercarriage overhead with noise and gusts of wind.

Investigators and analysts said runway selection and wind conditions may be central to understanding how the approach played out. Runway 29, where the plane landed, is described as the shortest runway at the airport at 6,726 feet (2,050 meters), and it is generally only used when strong winds are present. An air traffic controller told pilots at the time that winds were gusting up to 31 mph (50 kph).

Steve Arroyo, a safety expert who said he landed on that same short runway many times during his career at United, said “It’s one of the most challenging approaches in the world,” and that “The margin of error is extremely low.” Arroyo also said in comments shared with the reporting that “A major catastrophe was avoided by feet,” explaining that if the plane had been about five feet lower, it could have been catastrophic.

Other aviation investigators suggested that the inquiry could examine factors that affected how the flight executed the approach. Jeff Guzzetti, a former crash investigator for the NTSB and the Federal Aviation Administration, said investigators are likely to look at whether fatigue could have been a factor after the long flight from Italy. Guzzetti’s comments reflected the possibility that an incident of this type can involve more than a single technical error.

The investigation is expected to focus on multiple layers of evidence. The NTSB directed United to preserve the cockpit voice and flight data recorders for the agency’s examination, and the airline said the pilots have been put on leave while the investigation is underway. The reporting also said the pilots’ damage report was not recorded because the crew opted to call the tower on the phone after landing instead of broadcasting the details over the radio, and that would be among the items investigators could examine for how information moved immediately after the impact.