U.S. intelligence believes many bomb builders are foreigners who enter Iraq from neighboring countries, including Saudi Arabia. The Saudis insist they are making the border more secure.
CBS News correspondent Sheila MacVicar went to see for herself.
Rolling toward the Saudi border with Iraq, the desert looks wide open. Up close, it's a different story.
It is an area that is effectively no man's land. If you're not a Saudi border guard, you're not supposed to be here.
The first line of defense is a 20-foot tall berm that runs for nearly 500 miles. To the south is Saudi Arabia and a six-mile wide no man's land. There's a fence and another berm there. On the other side is Iraq.
Critics, including some U.S. administration officials, charge that the border is porous and a route for Saudi militants to join anti-American insurgents in Iraq.
The Saudis say they are working very hard to keep it sealed. They took CBSNews to the border for an exclusive look.
Captain Ahmed Nazzad Alanezi and his men patrol the section 24 hours a day.
"If there's any footprints here, it's easy for them to see," he says.
Border guards drag the desert with tires, then send scouts out to look for suspect footprints – a new use of tracking skills that go back to their Bedouin past.
But infiltrators keep trying.
Captain Alanezi says that smugglers wear sheepskin booties on their feet. The fur is supposed to wipe the footprints away.
On one occasion, the wolves in sheepskin slippers turned out to be Iraqi drug smugglers heading into Saudi Arabia.
But the U.S. government is more interested in pushing Saudi authorities to make it as tough as possible for would-be jihadis to get through this border to Iraq.
That battle is becoming increasingly high-tech.
Most infiltrators try to slip across the border at night. As darkness falls, the Saudis now scan with thermal imaging systems that can see miles into the desert. The human body shows up like a neon sign.
You can even see the detail on their clothes.
The government here says it has spent a billion dollars trying to make this stretch of sand impenetrable. More work is on the way. There are plans for a new fence equipped with concertina wire and electronic motion detectors.
"We hope that we secure the border 100 percent," Captain Alanezi says.
The Saudis say the real improvement is needed on the other side of the berm where the looted Iraqi border post tells the story. There are very few Iraqi or coalition patrols, they say. Until someone fills the vacuum on that side, this stretch of desert will remain a trouble spot.