A roadside blast kills eight people as the U.S. defense secretary visits Afghanistan A U.S. soldier is in custody in connection with the shootings Leon Panetta is the first high-ranking U.S. official to visit the country since the attack Camp Bastion, Afghanistan (CNN) -- U.S. Defense Secretary Leon Panetta flew into Afghanistan on Wednesday, making him the first high-ranking American official to visit the country since a U.S. soldier was detained on suspicion of killing 16 Afghan men, women and children in their homes Sunday. Panetta is making a two-day visit amid Afghan anger about the weekend shooting rampage. The defense secretary is due to meet with Afghan tribal leaders and government ministers, but his schedule does not include a trip to the area of Kandahar province where the killings took place. Also Wednesday, a huge explosion killed eight people in Helmand province, which Panetta was visiting, but he was not affected. The massive roadside blast ripped through a minivan, leaving its victims unrecognizable, the Helmand government said. The government blamed "terrorists" for the bombing. In a separate incident, a motorcycle bomb exploded in Kandahar, killing an Afghan intelligence agency security guard who was trying to defuse it, a government media agency said. Three others, including a civilian, were injured. It was unclear if the bombing was related to Sunday's killings. The Taliban have threatened to behead Americans in response to the shootings, and Afghan officials have expressed outrage about them, with President Hamid Karzai calling them "acts of terror and unforgivable." Despite some protests and local violence, the country has not erupted the way it did last month when American troops burned copies of the Quran and other Islamic religious materials. Military officials said the materials had been seized from Afghan prisoners because they contained extremist messages. President Barack Obama called that incident a mistake and apologized for it, but those efforts did not head off riots and reprisal attacks that left at least six American troops and dozens of Afghans dead. Panetta's first stop in Afghanistan was at Camp Bastion and the adjoining base Camp Leatherneck, where he was to meet with top U.S. and British military leaders and speak to a group of about 200 troops. From there, he will travel to a nearby forward operating base to meet more coalition troops before flying to Kabul. In Washington, Obama said Tuesday that American officials were "heartbroken" by the deaths but had no plans to change course in the decade-old war in Afghanistan. Sunday's pre-dawn rampage, which left nine children, three women and four men dead in two villages in rural Kandahar province, has added to the strain between Washington and Kabul. The Taliban have battled U.S. and NATO troops as well as Afghan government forces since the 2001 invasion after the September 11 attacks. Following Sunday's killings, they described U.S. troops as "sick-minded American savages." The U.S. military said the Army sergeant blamed for the killings acted alone. Two senior military officials told CNN that images from security cameras around his outpost showed the suspect leaving and returning alone to the base. There is another image showing him lying prone in a field. He then is shown getting up and walking back to the outpost, according to a senior U.S. military official with direct knowledge of the information. It is not clear how the images were taken. A military search party was put together to look for the missing soldier, but it's unclear to what extent searchers got under way before he returned to the base, the official said. The soldier, whom the military has not named, has yet to be charged. Military authorities have presented a determination of probable cause to allow them to keep the sergeant in detention, an International Security Assistance Force official told CNN. Investigators are looking into whether alcohol may have been a factor in the attack. Toxicology tests on the suspect are not complete, the two senior military officials said. Speaking Tuesday at the White House, Obama said he has told Karzai that the United States "takes this as seriously as if it was our own citizens and our own children who were murdered." CNN's Larry Shaughnessy and Sara Sidner and journalist Ruhullah Khapalwak contributed to this report.