(CNN) -- Tracy Morgan remained in critical condition Sunday but appeared to show signs of improvement after a deadly car wreck a day earlier, the comedian's publicist said.
"He has been more responsive today, which is an incredibly encouraging sign," Lewis Kay said.
Morgan's limo van was hit by a tractor trailer on the New Jersey Turnpike at about 1 a.m. Saturday, according to Sgt. Gregory Williams of New Jersey State Police.
Morgan suffered several injuries, including broken ribs, a broken nose, a broken leg and a broken femur. He underwent surgery on his leg on Sunday, Kay said, noting any road to recovery would be a long one.
"We expect him to remain in the hospital for several weeks," he said.
The chain-reaction wreck killed comedian James McNair -- who performed under the name Jimmy Mack -- who was riding with Morgan, and injured several others, according to officials.
The truck driver charged in the crash has turned himself in, police said Sunday.
Kevin Roper, 35, posted a $50,000 bail Saturday night, according to James O'Neill, spokesman for the Middlesex, New Jersey, district attorney's office.
Comedians Ardie Fuqua and Harris Stanton were among the injured, as was Jeffrey Millea, Williams said. One person was released Saturday, but Robert Wood Johnson University Hospital in New Jersey wouldn't disclose that person's name.
Truck driver Kevin Roper, 35, was charged with one count of death by auto and four counts of assault by auto.
Two others remained in critical condition Sunday, hospital spokeswoman Zenaida Mendez said.
Roper is charged with one count of death by auto and four counts of assault by auto after the tractor-trailer he was driving crashed into the limo bus, a statement from Middlesex County Prosecutor's Office said.
Walmart spokeswoman Brooke Buchanan said Roper is an employee.
"This is a tragedy, and we are profoundly sorry that one of our trucks was involved," Bill Simon, president and CEO of Walmart U.S., said in a statement.
"The facts are continuing to unfold. If it's determined that our truck caused the accident, Walmart will take full responsibility."
The wreck occurred about 1 a.m. ET Saturday on the turnpike in Mercer County, Williams said.
"The driver of the tractor-trailer failed to observe slow-moving traffic ahead," Williams said. "At the last minute, he swerved to try and avoid the Mercedes limo bus but struck it from behind, forcing the limo to rotate and overturn."
Four other vehicles were involved in the crash, the prosecutor's office said, but no one in those vehicles was injured.
Williams said he does not believe alcohol played a role in the crash, but that is under investigation.
The National Transportation Safety Board will investigate "safety issues related to commercial trucking and limousine safety," said agency spokesman Keith Holloway.
Christopher Thompson, the co-owner of Atlantic Transportation Services, which was operating the limo bus, said the driver of the vehicle did not want to comment on the wreck "while those who are injured are still fighting for their lives." He said he thought the bus was heading to drop Morgan off in New Jersey.
James "Jimmy Mack" McNair and Tracy Morgan "go way back," McNair's sister says.
Morgan, 45, was a regular on "Saturday Night Live" for seven years. He later received an Emmy nomination for his role in "30 Rock," in which he played Tracy Jordan, an exaggerated version of himself.
The New York City native made his reputation with over-the-top characters and impressions of celebrities such as Mike Tyson and Oprah Winfrey.
Morgan performed a comedy tour act Friday night at the Dover Downs Hotel & Casino in Dover, Delaware, the venue said.
The actor received messages of support and concern on Twitter.
"Wishing a speedy recovery to our friend @RealTracyMorgan," tweeted SNL Weekend Update.
"Shocked to wake up and read that @RealTracyMorgan is injured. Love to Tracy and wishing him strength," said former SNL cast member Rachel Dratch.
"My thoughts are with Tracy Morgan and his family. I hope he pulls through this soon and gets back to making us laugh!" said Joan Rivers.
"Saying a prayer for @RealTracyMorgan," said actor-director Jon Favreau.
"Come on @RealTracyMorgan pull through this buddy," tweeted actor Ike Barinholtz of "The Mindy Project."
Funny man Tracy Morgan has known fame and foibles