BRIDGEPORT — Investigators have determined that alcohol was not a factor in the accident that killed four Yale students Jan. 17, National Transportation Safety Board officials said Friday. Inadequate highway lighting was a possible factor in the crash, investigators said at a press conference in Bridgeport.

NTSB Investigator-in-Charge David S. Rayburn said preliminary test results indicated no evidence that the driver of the sport utility vehicle carrying the nine Yale students was intoxicated. Neither of the two truck drivers involved in the accident tested positive for alcohol or drugs, officials said.

The accident on I-95, in which a 1999 Chevrolet Tahoe crashed into a tractor-trailer, claimed the lives of four of the nine Yale students in the vehicle. Kyle Burnat ’05, Andrew Dwyer ’05 and Sean Fenton ’04 were pronounced dead at the scene, and Nicholas Grass ’05 died Saturday at St. Vincent’s Medical Center in Bridgeport.

Rayburn said poor lighting of Interstate 95 could have been a factor, since lights on the interstate were not working correctly at the time of the accident.

“The lighting was not working in the southbound and northbound lanes within the vicinity of the accident,” Rayburn said. “A construction accident caused the southbound lane’s lights to go out at about 1:30 a.m. [the morning of the accident].”

Officials said the construction company was working on replacing old median barriers on I-95 with taller, stronger barriers.

The construction company did not notify the state Department of Transportation about the malfunctioning lights, Rayburn said.

Officials said the reason that the northbound lane’s lights were inoperative has not been determined.

Driver fatigue was also investigated as a possible cause of the accident. Officials said, however, that Armando Salgado, 33, the driver of the Arrowhead Trucking Company freightliner that caused the accident, got a sufficient amount of sleep prior to the accident.

“He [the truck driver] had a seven-hour sleep period that happened three hours before the accident,” Rayburn said. But, Rayburn said, the driver of the SUV, Fenton, was “up all night before the accident.”

Rayburn affirmed the safety record of Arrow Trucking, saying the company has a satisfactory accident ratio.

Officials at the press conference said road conditions could also have been a factor in the accident because another accident within the vicinity of the fatal crash occurred just one hour earlier. Rayburn said Salgado reported in an interview conducted as part of the investigation that black ice — thin ice that takes on the color of underlying pavement — could have caused his truck to careen out of control, since it is not easily distinguishable from the roadway itself.

However, Rayburn said, regular de-icing was conducted on the interstate the morning of the crash.

“Trucks did in fact salt and sand I-95 within the vicinity of the accident between 1:30 a.m. and 4:30 a.m.,” Rayburn said.

Excessive speed was discounted as a cause of the accident, officials said.

“There is not any grossly excessive speeds involved,” Rayburn said. “The Tahoe was not grossly exceeding the speed limit.”

Rayburn also said post-crash inspections showed the vehicles involved had no defects that could have contributed to the accident. However, the SUV was carrying nine passengers — four passengers over the limit for which it was designed, Rayburn said. Both the driver and the front passenger wore seatbelts, but the seven passengers in the middle and rear of the vehicle did not, Rayburn said. However, officials said seatbelts would probably not have made a difference in the outcome of the accident.

Eric Wenzel ’03, who was injured in the accident, was last reported Thursday evening to be in critical condition at Bridgeport Hospital. Brett Smith ’06 remained in serious condition Friday evening in Norwalk Hospital’s intensive care unit. Chris Gary ’06 and Cameron Fine ’06, who were also injured, were released from Undergraduate Health Services Wednesday.