STORY: As the 20th anniversary of the U.S. invasion of Iraq draws near, Reuters visited a small forensics team tasked with untangling one of the biggest missing persons files in the world.

You're looking at the artifacts of decades of dictator Saddam Hussein's regime, wars involving the U.S., Iran, Al Qaeda, Islamic State, and other violence.

Yasmine Siddiq heads a group of about 100 people in the Ministry of Health which sorts mass graves, one site at a time.

"This unit is responsible for receiving the remains and cleaning them. The cleaning process starts with cleaning the clothes to get a better look at the clues. It also washes and cleans the remains to make the second examination phase easier.”

"More than 250,000 missing persons are officially registered with the government. The number that we physically received - that were unburied and examined - is more than 4,000 remains. So the number is really low when compared to the number of government registrations. And I want to make clear, the numbers registered by the government are approximate and are not actual.”

Siddiq says priority is given to the massacres committed by Islamic State, during its three years controlling parts of northern Iraq.

This is Hazem Mohammed. His missing person report goes back much further though, to the end of the Gulf War in 1991, when he and his brother were shot in a mass execution during a failed uprising against Saddam Hussein.

Here's the site today. Mohammed survived but his brother did not.

“I can't describe it. I felt getting hit by gunfire and fell into the pit. Two bullets went through my chest, one entered through here and exited from there. Another went through here.”

“We got out, we were moving from one pit to another, and in every pit we found a mass grave. We decided to move to a farm close to the highway."

"But my brother died. We did not manage to get him."

This is human bone, Mohammed says. He kept the location of this site secret for ten years after Hussein's fall in 2003, hoping for a more stable government to arise.

Eventually he brought a team of experts here. They found the remains of 46 people.

His brother was not one of them, but Mohammed thinks there are still more to be found.