It was the Fourth of July, Independence Day, when a terrorist breached the perimeter of an US base in eastern Afghanistan. The suicide bomber was driving a truck filled with explosives. It detonated just outside the gates of the base, killing Aaron Fairbairn, 21, and another officer, according to David Masters, Fairbairn's stepfather from Olympia, Washington. "There was a knock at the door," Masters told The Jerusalem Post on Sunday. "I was on the phone with one of my other sons, when I looked out the door and saw two full-dress officers on the porch." When the family was told the news of Fairbairn's death, they were in a state of disbelief. "We thought, oh, God, this couldn't be," Masters said. "We just talked to him last night. He said he never leaves the base, is playing video games, watching TVâ€¦ and working out with free weights." He and his wife Shelley began contacting relatives and dialing any person's number they could think of to contact, when they realized they had no way of reaching Fairbairn's friends. They did not know the passwords to his e-mail or MySpace accounts, which were his main means of communicating with them. So Masters began posting notice of Fairbairn's death on Twitter, the social networking site: "They killed my son, Aaron!" he wrote. He continued to vent his frustration on the site, he said, soon realizing that he could use the network of people to commemorate his son's death. "Thank you, Aaron, for making the ultimate sacrifice this Independence Day," Masters wrote, later asking the Twitter community to r-post "Thank you, Aaron" on their personal pages, in order to make the issue a popular topic on the Web site. And they did. On Sunday, it was ranked first under the site's trending topics section, a list of the day's most posted subjects. Fairbairn was scheduled to return home from his deployment Afghanistan in September. Members of his family were awaiting the arrival of his body to US soil in Dover, Delaware on Sunday. "We all miss him so much, and this was so unexpected," Masters said. "I mean, it could happen to anyone at any time, but Aaron is a very do-the-right-thing young man with a huge heart-component, who loved deeply and shared his life openly with so many. His life is led by God, strength and honor." Responding to the deteriorating security situation, President Barack Obama's administration has ordered 21,000 additional troops to Afghanistan and expects the total number of US forces there to reach 68,000 by year's end. That is double the number of troops in Afghanistan in 2008 but still half as many as are now in Iraq. As part of the new strategy, 4,000 Marines poured into volatile Helmand province on Thursday in the biggest US military operation in Afghanistan since 2001, trying to cut insurgent supply lines and win over local elders. Saturday's attack happened in the same province where an American soldier and three Afghans were believed captured by insurgents Tuesday. US troops continued looking for the soldier, Navy Chief Petty Officer Brian Naranjo said Friday. The military has not publicly identified him. No immediate claim of responsibility was made by any insurgent group for the missing soldier or Saturday's attack. AP contributed to this report.