WASHINGTON — Rondell Henry knew nothing about guns or explosives, federal prosecutors say, but he knew how to drive and that was enough.

One day last month, the 28-year-old computer engineer abruptly walked out of his job in Maryland, stole a U-Haul van he spotted on the highway and started driving through the Washington suburbs looking for places where he could run down crowds of people. Nothing seemed to his liking. Eventually, he parked at a tourist spot on the Potomac River called National Harbor and waited, according to court documents.

His plan, after two years of following the Islamic State on the internet, was to harm "disbelievers," he said.

"I was just going to keep driving and driving and driving," Henry told investigators, according to court records. "I wasn't going to stop."

His plan wasn't sophisticated, or much of a plan at all. Local police found the stolen truck, waited for him to return and arrested him.

But experts said it is the type of plan – lone-wolves who are radicalized online and need little in the way of tools or training – that are the most difficult types of terrorist attacks to prevent.

RELATED: The making of an American terrorist: Hoda Muthana joined ISIS. Now she can’t come back

RELATED:Should Hoda Muthana be allowed to return from ISIS for trial in the U.S.?

"Because there's not a real conspiracy. If the individual is not boasting on the internet that they're going to do something, in other words, not attracting attention to themselves, then there's no indicator," said Brian Jenkins, director of the Mineta Transportation Institute’s National Transportation Safety and Security Center, and an expert on terrorists targeting soft targets such as malls or train stations. "There's no alarm bell to warn authorities that something was up."

Car and truck attacks require little skill or planning, and they are becoming a more common method for terror attacks in the United States and Western Europe. Terrorists who use trucks don't have to find explosives or figure out how to assemble a bomb. Anyone with a vehicle can drive into a crowd anywhere, and can do so with very little forethought, Jenkins said.

An analysis by the Mineta Transportation Institute's National Transportation Safety and Security Center found that of the nearly 80 car-ramming attacks that target random groups of people over the past four decades, 30 happened in 2017 and in the first four months of 2018. The analysis covered incidents in 19 countries, including the United States, Canada, Australia, China, Japan and Western European countries.

"What we're seeing is an increase in the number of homegrown terrorists using very, very primitive tactics against random targets ... The reason we're seeing this is there is an exhortation from these jihadist groups like Al Qaeda, like ISIS, to inspire homegrown terrorist to do whatever they can, where ever they are," Jenkins said.

"They're extremely difficult to prevent. It's a single individual. There's no intelligence system," Jenkins added. "There's no radar for a man's soul."

In Henry's case, the authorities said he had harbored hatred against people who don't practice Islam. He watched videos of foreign terrorists beheading civilians overseas and sought to emulate them, according to a motion to keep him detained pending trial. He also knew of the terrorist attack in Nice, France, where a terrorist drove through a crowd, killing dozens of people, and wanted to copy that.

So far, Henry is charged only with transporting the stolen U-Haul into Maryland. But in court documents filed on Monday, prosecutors in Maryland laid out a far more serious crime.

Authorities say Henry left his job in Germantown, Maryland on Tuesday and drove around the Washington metropolitan area in his four-door sedan looking for the vehicle large enough to inflict the most damage. He spotted a U-haul van in Alexandria, Virginia, just outside of Washington, followed it to its storage location, and stole it. He drove around, assessing targets that would attract the most media coverage, court records say.

Henry wanted to create "panic and chaos," he told investigators — like "what happened in France."

He settled for Dulles International Airport, where he arrived early morning on Wednesday. There were no crowds at the airport then, so Henry, authorities say, got out of the van and tried to get through airport security. He was unsuccessful. So he went back to the van and drove more than 30 miles to the National Harbor in Maryland, where he waited for a larger crowd.

Henry broke into a nearby boat and stayed there overnight, court records say. The following morning, police had found the stolen U-Haul and were waiting for Henry. Police had traced him through his car, a blue BMW he left at the same parking garage where he stole the van.

Henry is scheduled for a detention hearing on Thursday.