The truck crash that killed 10 teenagers in Anne Arundel County last April 1 probably was caused by highspeed and reckless driving by a driver who was under the influence of alcohol and marijuana, the National Transportation Safety Board said yesterday.

The board also found that the crowding of passengers into the unprotected open bed of driver Alan Cole's pickup truck was responsible for the severity of the accident, one of the worst, in Maryland history.

As a result of the accident investigation, the federal agency recommended that laws be adopted to prohibit passengers riding "in open-cargo areas of vehicles not used for work purposes."

The safety board, which normally looks into airplane accidents, is legally required to investigate highway accidents involving five or more fatalities.

Board spokesman Bob Buckhorn said the investigators "interviewed extensively" both 18-year-old Cole and witnesses who said they saw Cole drink tequila and beer and smoke marijuana on the day of the crash.

The board accepted a report by its staff yesterday. The report said blood tests showed Cole had a blood alcohol level of .135 near the time of the crash. That was the equivalent of drinking seven one-ounce shots of 80 proof alcohol, the report said.

Cole has been charged with automobile manslaughter and homicide with a motor vehicle while intoxicated in each of the deaths. He also was charged with one count each of driving while intoxicated, speeding, and reckless driving. Trial is set for October in Frederick County.

On the evening of April 23, Cole's 1978 Ford Courier ran off a winding rural road, struck three trees and turned over. Seven of the 12 passengers were killed instantly, and three died later. Cole escaped with slight injuries, while the other survivor was more seriously hurt.

Board investigators and tire marks on the road indicated the truck had been going between 64 and 78 mph at the time of the crash.