A small jet that crashed into a building in Connecticut in 2021, killing four people, was going slower than usual as it took off because the pilot had left a parking brake on, according to a report released Thursday.

The failure to release the brake was the probable cause of the accident, according to the report from the National Transportation Safety Board, which also said a lack of equipment on the jet to notify the pilots of such a problem contributed to the crash.

The twin-engine Cessna 560XL was to have flown from Robertson Airport in Plainville to Dare County Regional Airport in Manteo, North Carolina, on Sept. 2, 2021. But it crashed into a manufacturing building shortly after takeoff and burst into flames.

A husband and wife who were both doctors, Courtney Haviland, 33, and William Shrauner, 32, of Boston, were killed along with the two pilots, William O’Leary, 55, of Bristol, and Mark Morrow, 57, of Danbury.

Four people on the ground were injured, including one who was seriously hurt. Haviland and Shrauner left behind a toddler, and Haviland was pregnant, according to relatives.

As the plane was accelerating on the runway, a witness noticed it was going slower than usual, the report said. A witness saw blue smoke from the jet’s rear, and another witness saw the nose landing gear still on the ground near the midpoint of the runway.

The jet hit a utility pole and a grassy area next to the Trumpf Inc. building near the runway. It then struck the building, causing a fire that consumed most of the aircraft, the report said.

Investigators noticed the parking brake was on and there were skid marks on the runway.

The NTSB report also faulted the certification process for the airplane, which allowed the jet to be manufactured in 2006 without a parking brake indicator because it was made as a “derivative” of an older model that used a standard from 1965 that did not require that type of warning equipment.