A sport utility vehicle carrying three people fell off the Chappaquiddick ferry into the frigid waters of Edgartown Harbor late Tuesday night, prompting a hasty rescue of both the passengers and vehicle.

Following the incident, police said they plan to issue a summons to the driver for negligent operation of a motor vehicle and operating under the influence.

Edgartown police first received a call that a car had fallen into the water just after 11 p.m. When they arrived on the scene, they found that a 2006 Mercedes-Benz sport utility vehicle had fallen off the ferry about 10 feet from the Chappy side of the harbor.

The driver, Susan McLean, 54, of Ambler, Pa., her daughter Audrey McLean, 19, also of Ambler, Pa., and Alexander Tasch, 23, of Nantucket, were all plunged into the waters, although none were seriously injured.

Sgt. Kenneth Johnson said all three passengers were already out of the water by the time police arrived. The sergeant said Susan McLean failed to put the vehicle in park after boarding the ferry and instead put the car in neutral.

“Mrs. McLean was moving the steering wheel back and forth and pretending she was driving the ferry,” Sergeant Johnson said. “During that time, she knocked out the wooden chocks from under the wheel of the vehicle. When the ferry approached the slip, the vehicle lurched forward and fell into the water.”

Sergeant Johnson said on Wednesday that Mrs. McLean likely would be cited for some type of motor vehicle violation. Later yesterday afternoon, Edgartown police chief Paul Condlin said that Mrs. McLean would be summonsed for negligent operation and driving under the influence.

The U.S. Coast Guard has also opened an investigation into the incident, police said.

The Chappy ferry at present is operating on a winter schedule, meaning the last trip from Edgartown to Chappaquiddick is at 11 p.m. The standard procedure is for cars to drive on one end, then off the other, and for a ferry employee to place wooden chocks under the vehicle’s wheels while a heavy chain is stretched across the front and back of the boat.

According to several eyewitness accounts, the captain of the boat, Brad Fligor, helped pull the three passengers out of the water and onto the ferry after the vehicle fell into the water. Mr. Tasch reportedly managed to roll down a window and extricate himself from the vehicle and helped the two other passengers to safety.

Steve Ewing, an Edgartown-based dock builder, used one of his company’s tow boats to pull the Mercedes-Benz to the shore. A diver from the Edgartown fire department reportedly attached a cable to the vehicle’s axle, and the tow boat dragged the vehicle about 150 feet along the ocean bottom to the edge of the inner harbor and close enough so a tow truck could attack a cable and pull it out.

Mr. Ewing said he could hardly believe it when he received the call around 11:30 p.m. that a car had fallen into the harbor.

“I was asleep in my pajamas when I got the call, and at first I thought it was a joke,” he said. “I can’t remember a car ever falling off the ferry. The whole thing was sort of unreal . . . when we were dragging the car towards the shore I could feel it skipping along on the bottom. It was kind of like fishing.”

The salt water reportedly damaged the computer system controlling the vehicles anti-lock brakes, which prevented it from being transported by regular tow truck. The vehicle was later hauled away by a flatbed truck operated by MV Auto Salvage.

Susan McLean contacted the Gazette Wednesday afternoon, before police said they planned to charge her with negligent operation of a motor vehicle and operating under the influenace, to refute what she described as an untrue rumor that she drove the sports utility vehicle off the ferry and into the water.

Although she stopped short of blaming Mr. Fligor for the incident, she suggested that he was not as helpful as possible.

Mrs. McLean said she told Mr. Fligor that she planned to drive over to Chappaquiddick and drop off Mr. Tasch at his family’s home, and then quickly return so the ferry could transport her back to the Edgartown side.

Mrs. McLean said Mr. Fligor seemed frustrated he would have to wait for her to return and said she couldn’t recall if he told her to put the car into park or whether he put the chocks under the tire. She said he did not tell her to turn the car’s ignition off.

Mrs. McLean also disputed the police account that she was moving the steering wheel and pretending to steer the ferry. She maintained she only grabbed the wheel after the ferry swerved suddenly. She also questioned why Mr. Fligor had her park at the front of the ferry when hers was the only vehicle on board.

Both Mr. Fligor and the owner of the Chappy ferry, Peter Wells, could not be reached for comment. Mr. Wells purchased the ferry line from former long-time owner Roy Hayes in January.

Edgartown harbormaster Charlie Blair said he doubted Mr. Fligor was to blame for the mishap.

“Mr. Fligor has been running that boat for a long time. He has a great safety record and is super-square when it comes to the rules. As far as I am concerned he is a hero for helping those people [out of the water],” he said.

Mr. Blair also offered a blunt assessment of the incident.

“It was very lucky that we weren’t fishing bodies out of the harbor . . . those people should consider themselves to be fortunate,” he said.