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Waco, Texas (CNN) -- A row of 12 coffins, each draped with a flag, symbolizes the human cost of an explosion that destroyed much of a small Texas town and left the residents of that town searching for answers.

Hundreds of firefighters from across the nation and other first-responders joined grieving families at the Ferrell Center in Waco, site of a memorial service Thursday for the people killed in a fertilizer plant explosion last week in the nearby town of West.

Everyone inside the arena stood in complete silence as family and friends of the victims filed in. Some of the family members wept as they passed the flag-draped coffins -- 10 with American flags and two with Texas flags -- and photos in front of each one.

Family members hugged one another when they arrived at their seats.

President Barack Obama and first lady Michelle Obama are on hand to join the residents of West at the service at Baylor University.

Emotions were raw in Waco, the central Texas city located about 20 miles south of the scene of the blast.

Before the service began many crying onlookers lined the street outside as a procession of fire trucks slowly passed. One man held a large Texas flag, bowed his head and rested a cowboy hat over his heart.

Hundreds of bikers from the Patriot Guard stood at attention as the rescue vehicles passed by.

Before leaving Washington, Obama signed a proclamation ordering all flags in the state to be flown at half-staff for the day.

City workers from Waco replaced West workers Thursday so they could attend funerals and take a break from trying to repair the city's water system and cleaning up the town.

On Wednesday, an American flag was raised and a bugler played taps at a memorial service at the blast site. A few miles away, one of the 14 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 fire truck to be carried to the cemetery. Bagpipes played as the coffin went through the crowd.

Before and after: Texas plant explosion

"Shovel by shovel," investigators are combing through the charred remains of the leveled fertilizer distributor after the April 17 explosion in West.

Much of the landscape surrounding the West Fertilizer Co. is unrecognizable. What was once a corn silo appears to have crumpled from the blast. A blue tarp covers the shell of a rail car.

A crater nearly 100 feet wide and 10 feet deep sits where a building once stood. 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.

Losses from the explosion will probably top $100 million, said Mark Hanna, a spokesman for the Insurance Council of Texas.

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.

Town devastated by explosion is guided by the West way

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."

Robert Champion, special agent in charge of the bureau's Dallas Field Division, 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," he 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.

CNN's Todd Sperry contributed from West, Texas, and Ed Payne wrote and reported from Atlanta. CNN's John Murgatroyd contributed to this report.

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assistant: Hundreds gather for memorial for victims of Texas plant explosion