One of the people accused by the FBI of being part of hacking group Lulz Security led a non-profit organisation in Galway, Ireland, dedicated to making websites more secure.
Darren Martyn is charged with computer hacker conspiracy and faces up to 20 years in prison.
A biopharmaceutical chemistry student at NUI Galway, Martyn was involved in the Open Web Application Security Project (OWASP), which develops open-source applications to improve security.
In an email to Boards.ie website after his indictment, he expressed regret for carrying out illegal hacking activity under the alias Infodox.
"Knew it would come out eventually. I suppose one cannot hide forever from their past. I had gone kinda-whitehat [an ethical hacker] just a small bit before my arrest, and since my arrest have worked to try 'rehabilitate' self by contributing as much as I can to the open source/security community while I remain at liberty to do so.
"As I have said in the past - going blackhat is probably the most stupid thing you can do," he adds. "I had to learn that the hard way."
"Black hat" is used to describe a hacker who breaks into a computer system illegally or with malicious intent.
The FBI claimed that Martyn - also known as "pwnsauce", "raepsauce" and "networkkitten" - launched cyber-attacks and got access to the websites and computers of the Fine Gael party, Sony, internet security firm HBGary and Fox broadcasting.
"It is a very upsetting time for the family," his mother told her local paper, the The Connacht Tribune.
Described as a quiet, shy and highly intelligent man by locals, Martyn has been honoured with awards in the past for his science and technology skills. On his Facebook page, the list of interest includes "hacking computers", "ethical hacking" and "lock picking".
Thomas Brennon, director of the OWASP, said Martyn resigned. "It's about laws and ethics and people have to determine whether they want to follow the speed limit, follow the law," Brennan told Reuters. "We have the same skillset as the bad guys, but the only difference is ethics."
Martyn is listed as residing in Ireland, but it was unclear if he had responded to the US charges.
The indictment says he is 25, but local Irish papers claim he is 19 or 20.
Martyn was arrested as part of a larger campaign by law enforcement officers that saw five alleged LulzSec members arrested around the globe on Tuesday - two in the UK, two in Ireland and one in Chicago. The operation follows the 2011 arrests of several other alleged LulzSec members including the group's supposed "PR man" Jake Davis, aka Topiary.
Though the two are closely linked, the LulzSec group is separate from the larger Anonymous collective. LulzSec started in 2011 with a more anarchist ideology, targeting random organisations and companies "just for lulz" - internet jargon for laughs. Anonymous portrays itself as a hacktivist collective, picking its targets for perceived crimes against the world.