(CNN) -- More members of the National Transportation Safety Board will arrive in Philadelphia, Pennsylvania, on Thursday morning to investigate a collision between a tour boat and a barge on the Delaware River a day earlier.
"We are here for fact finding," said Robert Sumwalt, NTSB spokesman, about a group that already arrived Wednesday night. "Our team will begin looking at any radio recordings, photographs that may have been taken by witnesses or people on board, and we will be conducting interviews with those on board."
The tour boat, popularly known as a "Duck" boat, overturned Wednesday when it was hit by the barge, according to the U.S. Coast Guard.
Thirty-seven passengers and crew members on the boat at the time have all been accounted for except for two, said Todd Gaitlin, U.S. Coast Guard captain.
"A 16-year-old young lady and a 20-year-old man are still missing," he said at a news conference in Philadelphia on Wednesday night.
"Our search efforts will continue overnight," he said. "Divers will resume their search in the water Thursday morning."
As many as 20 vessels and six helicopters were involved in the search Wednesday, Gaitlin said.
Video: Witness watched barge hit boat
Video: Two missing in Delaware River accident
Video: Boat accident strands passengers
U.S. National Transportation Safety Board
U.S. Coast Guard
Searchers located the boat in about 50 feet of murky water, but it was too dangerous to send divers down, said William Blackburn, deputy commissioner of the Philadelphia police.
The boat is upright on the bottom of the river, Blackburn said, adding that it has been marked by a buoy and divers will be sent down when it is safe.
The "Duck" boats are amphibious military vehicles designed in the 1940s. The boat involved in the Philadelphia crash was operated by "Ride the Ducks of Philadelphia," Sumwalt said.
"We will be looking at when the boat was manufactured, if it was refurbished, how often it was inspected," he said.
The NTSB will also conduct a 72-hour history to determine the mental and emotional state of the crew of the boat vessels, he said.
Lt. Frank Vanore of the Philadelphia Police Department told reporters that the boat had "mechanical trouble" and the engine shut down after a fire on board.
While the boat was in the river and waiting for help, it was hit by a barge, he said.
Hahnemann University Hospital reported eight patients -- two adults, three children and three teenagers -- were brought in after the accident. Two other passengers on the boat were also taken to the hospital but declined treatment, said Coleen Cannon, hospital spokeswoman.
Of the eight patients, one adult and two children were discharged a few hours later, she said.