West, Texas (CNN) -- "Shovel by shovel," investigators are combing through the charred remains of a fertilizer distributor leveled by a massive explosion one week ago.
Much of the landscape surrounding the West Fertilizer Co. in West, Texas, is unrecognizable. What was once a corn silo appears to have crumpled in the force of the blast. A blue tarp covers the shell of a rail car.
Left is a crater nearly 100 feet wide and 10 feet deep that was once the site of a building. The crater is now filled with mangled metal and crumbs of mortar. Concrete chunks -- some the size of shopping carts -- are strewn hundreds of yards away from the blast site.
But amid the devastation, forensic mappers are hoping to find clues.
Hundreds of small pink flags indicate anything on the ground that crews want forensic investigators to take a closer look at.
Officials face a difficult task in reconstructing the fire that preceded the deadly explosion. Still unknown: what types of chemicals and in what quantities were stored at the facility.
One official likened the investigation to a jigsaw puzzle.
"Right now, think of that coffee table where all 100 pieces are gathered around," Brian Hoback, an official with the Bureau of Alcohol, Tobacco, Firearms and Explosives, told the San-Antonio Express News. "Now, we're going to pull them together."
ATF Special Agent in Charge Robert Champion said determining what started the initial fire is "the key."
"We feel the explosion was caused by the fire so we've got to determine what the cause and the origin of the fire was, and that's why we're ... attempting to re-enact that fire scene," Champion said. "A fire scene is complicated in itself. But you compound that with an explosion and it really complicates the issue."
So far, investigators have ruled out the possibility that natural causes ignited the fire.
Meanwhile Wednesday, in a memorial at the blast site, an American flag was raised and a bugler played taps as workers paused to remember the 14 people killed. A few miles away, one of those victims, Kenneth "Luckey" Harris Jr., was laid to rest. Harris, a 52-year-old Dallas firefighter who lived in West, was one of 10 first responders who died.
Hundreds of firefighters from Dallas and other areas surrounding West came for the funeral, the first to be held for a first responder killed.
Firefighters lined the sidewalk as Harris' flag-draped coffin was carried out and loaded into the back of a Dallas firetruck to be carried to the cemetery. Bagpipes played as the coffin proceeded through the crowd.
President Barack Obama and first lady Michelle Obama will attend a larger memorial service on Thursday in Waco for all 14 victims. Ahead of the service, the president signed a proclamation Wednesday ordering all flags in the state flown at half-staff on Thursday.
City workers from Waco will replace West workers on Thursday so they may attend funerals and take a break from trying to repair the city's water system and cleaning up the town.
CNN's John Murgatroyd contributed to this report.