Were the terrorists who torched 14 tractor-trailer rigs at Harris Farms in California’s Central Valley unaware that the drivers of those vehicles sometimes slept overnight in those rigs?

Or was surveillance by those terrorists so pronounced that they knew nobody was in those rigs in the early morning hours as they set fire to them on Jan 8, 2012.

Either scenario is chilling to Tom Knowles, a retired agent with the Federal Bureau of Investigation and a terrorism authority with the Sacramento Regional Terrorism Threat Assessment Center.

Knowles was among three speakers at an ag crimes and terrorism summit who warned that the central San Joaquin Valley, boasting billions of dollars in agricultural production, can be vulnerable to attack from terrorists from abroad or homegrown.

Their observations came at the Kearney Ag Center in Parlier at an event presented by the University of California Cooperative Extension, Fresno County Farm Bureau, Fresno County Sheriff’s Office and the Fresno County Department of Agriculture.

A Kingsburg native, Knowles said he has been involved in investigations of all terrorist bombing attacks against the United States since the first Trade Center bombing in 1993, with the exception of the bombing of the federal courthouse in Oklahoma City.

Both Knowles and Fresno County Sheriff Margaret Mims conceded that investigation of terrorist actions is complicated. “They require marathon investigations,” Knowles said. But he believes if suspicious activity on Valley farms is reported, those responsible can “easily be caught.”

There have been no arrests in connection with the Harris Ranch arson that caused more than $2 million in damage.

Both Knowles and Mims showed a message posted on the Web by the North American Animal Liberation Front. It closed with the jarring phrase: “Until next time . . .”

“It leaves no doubt they’re coming back,” Knowles said. “It’s game on time.”

He believes a key to stopping such actions — and perhaps finding those who did the damage at Harris Farms — lies in heightened awareness among growers and field workers and paying more attention to what could be suspicious activity.

(For more on agroterrorism awareness and prevention, see: US agriculture waking up to agroterrorism threat)

Knowles, who is with the Sacramento Regional Terrorism Threat Assessment Center, said agriculture is particularly vulnerable to attack because “you don’t have to touch it to cause problems.” It can be enough to simply feed into a perception that food is contaminated.

Knowles cited the example of a food safety scare in 1996 that cost California’s strawberry industry $11 million dollars, despite the fact that the parasite that was making some ill was subsequently traced to raspberries from Guatemala.

Likewise, cherry tomatoes were the casualty in more recent years of a food safety scare triggered by peppers from Mexico.