The smoke has cleared but the mystery remains.

Safety officials say it may take several days to determine what caused the explosion Wednesday morning that destroyed a building at Trojan Corp.'s powder plant, injured seven workers, broke windows as far as five miles away and awakened residents 10 miles away."When you have an explosion, you basically lose most of your evidence," said Utah County Sheriff's Capt. Owen Quarnberg. "You theorize and can get pretty close to an explanation, but unless there are witnesses you never know for sure."

There was a busload of witnesses to the incident at 6:37 a.m. Workers had just taken a break and were headed back to Building 20 when the blast occurred. Seven riders suffered minor injuries and the bus was damaged by the explosion 50 yards away.

Two workers were treated at the scene, and five were taken to Mountain View Hospital in Payson. Hospital spokeswoman Pam White identified those five as Larry Lewis, 40, Spanish Fork, who suffered a jaw injury; Greg Marshall, 25, and Don Aitken, 25, both of Spanish Fork, back pain; David Hall, 34, Springville, elbow injury; and Leonard Steggell, 41, Spanish Fork, minor head trauma.

Steggell told reporters: "They (company officials) don't want us saying a whole lot about the accident. I am not opening my mouth."

Trojan employees Jerry Purkey, Spanish Fork, and Dennis Pearson, Payson, were treated at the company facility. Pearson later went to Mountain View to be examined for blunt head trauma and dizziness. He was treated and released.

"(The Trojan employees) were all really lucky," White said. "If they had been in the building when it exploded there would have been a lot of fatalities."

Quarnberg said county, state and federal officials will help Trojan investigate the accident, but "some of the best engineers around work for Trojan or are being brought in by the company."

The first task will be determining where the blast occurred, he said. In addition to the physical evidence, such as where the crater is deepest, officials will interview witnesses.

"Then investigators will determine what was in that area to see what was possible. From then on, it's just a process of elimination to see what reaction was most likely," he said. There is no evidence the explosion was caused by criminal activity, he added.

Sheriff's officials said Building 20 is used to manufacture "boosters" and "primaries," which contain TNT. Boosters and primaries are used as dynamite replacements. Damage to the plant was extensive; three other structures also were damaged.

Spanish Fork Fire Chief Lloyd Miller said four trucks were called to the site to stand by, but no major fires broke out.

"The explosion destroyed the building. There wasn't really much left to burn," he said.

No one but a few Trojan workers got close enough to see the damage, Miller said.

"All the explosives buildings are built into hills and are around a corner from the rest of the complex. They are separated for safety purposes."

Quarnberg said the company kept people away from the affected area. "In a case like this, some materials explode and some are just thrown to other areas. It would be dangerous to let people walk around before everything was cleaned up."

Although plant repairs may take a while, Quarnberg said, the area is "stable" and there is no reason for area residents to expect another explosion. Company officials have controlled risks by limiting access to the area, turning power off, removing damaged explosives and beginning repairs on damaged equipment.

But residents are still jumpy. A sonic boom at about 8:30 p.m. Wednesday sparked dozens of calls to public safety officials and the Trojan Corp. Many Provo and Orem residents thought there had been another explosion.

John Feasler, Trojan's executive vice president and chief operating officer, said he hopes the company can put some of its 150 workers back to work on unaffected projects within the next few days. He said the company hopes to release information later this week on the probable cause of the explosion.

There are no official estimates yet on damage to nearby businesses and homes, but Feasler said his company plans to bring in adjustors to determine the cost of necessary repairs.