Obama to attend memorial for victims of Texas plant explosion By Todd Sperry and Ed Payne, CNN April 25, 2013 -- Updated 0540 GMT (1340 HKT) The White House orders flags in Texas to be flown at half-staff Officials are combing through rubble to find source of initial fire The types of chemicals stored at the plant are not yet known Firefighter Kenneth "Luckey" Harris Jr. is laid to rest West, Texas (CNN) -- In a central Texas town still looking for answers, President Barack Obama will join the community of West on Thursday for a memorial service for the 14 people killed in a fertilizer plant explosion last week. First lady Michelle Obama will also attend the service at Baylor University in nearby Waco, Texas. 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 will replace West workers Thursday so they can 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 in a memorial service at the blast site. A few miles away, one of the 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. The investigation continues "Shovel by shovel," investigators are combing through the charred remains of the leveled fertilizer distributor following 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 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. Town devastated by explosion is guided by the West way Putting the pieces together 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. CNN's Todd Sperry contributed from West, Texas, and Ed Payne wrote and reported from Atlanta. CNN's John Murgatroyd contributed to this report.