ST. LOUIS — Roughly 600,000 people in the U.S. die every year from heart disease. It is the country’s leading cause of death.

It is also one of the most baffling. About half of heart disease victims die suddenly without showing any symptoms.

For more than a decade Dr. Pamela Woodard, a radiologist at Washington University’s School of Medicine, along with other researchers around the country, have been working on developing a procedure that could serve as an early warning system for people at risk of having a stroke or a sudden heart attack.

What they’ve come up with is a minuscule particle, known as a nanoparticle, that once injected into the body, can seek out and identify the harmful buildups in arteries that can rupture and lead to sudden death.

After 10 years of injecting the nanoparticle into mice with no harmful side effects, the Food and Drug Administration recently cleared Woodard and her team to start testing on human subjects.

"This is huge," Woodard said. "If everything goes well, hopefully we could get this to the market in two years."

For Woodard, the journey started about 14 years ago during a conversation with her brother, Geoffrey Woodard, a science researcher in Maryland.

He was studying the causes of high-blood pressure when he mentioned a certain protein found in the body responsible for how plaque develops.

Plaque buildup made of calcium, fat, cholesterol and other substances causes arteries to narrow and harden. Pamela Woodard explains that certain plaque buildups are prone to rupturing, causing strokes or heart attacks.

"That’s what we were looking at," she said. "The kind of plaque deposits that are more likely to kill you unexpectedly."

She said that veteran Washington, D.C., journalist and "Meet the Press" moderator Tim Russert has become something of an unofficial poster child for this type of death.

Russert, 58, had passed a stress test and was determined by his doctors to have good heart function just two months before he suffered a plaque rupture that led to his fatal heart attack in June 2008.

Woodard, along with researchers at the University of California, Santa Barbara and Texas A&M University, combined different materials commonly used in medicine to create the nanoparticle they believe can be used in the future to help identify people at risk of plaque rupture.

A nanoparticle is small enough that more than a billion can fit on the head of a pin. The one created by Woodard and her colleagues can be injected into a patient’s vein. Once inside the body, it will seek out and "light up" the type of fatty plaque that is prone to rupture.

A doctor can then put the patient through a positron emission tomography, or PET, scan to identify the plaque buildups.

Woodard, her brother and two other researchers now hold a patent on this particular nanoparticle.

Since getting FDA approval, the nanoparticle has been injected into two healthy volunteers, with no apparent side effects, Woodard said. Researchers expect to inject the nanoparticle into two other healthy volunteers within a month.

"Hopefully in March, we’ll be able to test this on actual patients," she said.

At least one member of the research team is anticipating other practical uses of the nanoparticle if the current tests go well. Craig Hawker, director of the California Nanosystems Institute at the University of California, Santa Barbara, believes the targeting properties of different nanoparticles can be assembled "like Lego building blocks."

In other words, the nanoparticle used to seek out the causes of heart disease can be re-engineered to target certain cancers.

It would be "easy to swap," he said.