A bus that crashed into a big-rig whose driver fell asleep during a freeway closure last year, killing 13 people, had no advance warning because state workers didn't properly plan for the closure, federal officials said Tuesday.

California Highway Patrol officers briefly stopped traffic on Interstate 10 near Palm Springs on Oct. 23, 2016, because of utility work. The truck was in a lane and didn't move again immediately.

A bus carrying people back from a desert casino rear-ended the big-rig at 76 mph, driving the trailer more than a dozen feet into the bus. The bus driver and a dozen passengers died and 30 passengers were injured.

Bruce Guilford, 51, of Georgia was charged earlier this month with 13 counts of vehicular manslaughter with gross negligence; 12 counts of felony reckless driving causing injury and 17 misdemeanor counts of reckless driving causing injury.

A CHP investigation concluded that the trucker didn't move for two minutes after traffic resumed flowing because he had fallen asleep after working illegally long hours.

The National Transportation Safety Board on Tuesday released a summary of its upcoming report on the crash.

It said the California Department of Transportation didn't have a proper traffic management plan for the road stoppage, which meant CHP officers didn't realize the truck hadn't begun moving and the bus driver had no advance warning.

In identifying safety issues, the NTSB report said Caltrans, which approves temporary traffic breaks, doesn't require law enforcement to use advance warning devices.

Having such devices and putting a CHP patrol car with flashing lights at the end of the traffic queue "could have alerted the bus driver about the traffic break, possibly increasing his vigilance sufficiently to enable him to detect the stopped truck in time to prevent the crash," according to the report summary.

Caltrans said in a statement that it has worked closely with the NTSB and CHP on their investigations, "which have concluded that the primary cause of the incident was a recurring problem on our nation's highways_truck driver fatigue."

"The NTSB has indicated that they will be looking into changes to laws and enforcement that will help save lives," the agency said. "Caltrans supports any effort to increase safety on our state's roadways."

The NTSB report noted that the trucker fell asleep during the traffic break but it said both drivers were fatigued.

The trucker had violated regulations about how long he could drive for days before the crash and also may have suffered from undiagnosed sleep apnea while the bus driver had gotten four hours of sleep or less in the day-and-a-half before the crash, the report said.

The bus driver also had untreated and poorly controlled diabetes although it's unclear whether he had blurred vision because of it, the report said.

The report did indicate that despite having no advance warning that the truck was stopped on the road, the bus driver had enough time to avoid an accident.

In both cases, the drivers had successfully completed federal medical examinations for their commercial driver's licenses, the NTSB indicated.

The report recommended better guidance from the Federal Motor Carrier Safety Administration, which certifies medical examiners for commercial driver's licenses.