Four men identified as members of the infamous LulzSec hacking group have pled guilty to illegal hacking dating back to 2011. Each of the four could wind up spending as much as 10 years in U.K. prison – though news reports suggest two to three years is a more likely sentence.

The latest of the quartet to enter a guilty plea was Ryan Ackroyd, 26. Known online as "Kayla," he was charged with carrying out an unauthorized act to impair the operation of a computer, according to news reports.

In 2011, LulzSec attacked websites of Sony, the FBI, the CIA, PBS and other organizations. The series of attacks lasted for several months.

“During that brief period it managed to wreak havoc on the internet and took over websites with abandonment,” according to a report on the group from The Inquirer.

Recently, LulzSec was described as the “hacking offshoot of Internet activists Anonymous,” Ars Technica reported. It is considered a loosely organized group, while Anonymous has more political objectives.

Mustafa Al-Bassam ("tflow”), 18, Jake Davis ("topiary"), 20, and Ryan Cleary, 21, previously pleaded guilty to charges in connection with the 2011 hacking spree.

Each defendant will be sentenced on May 14 in London's Southwark Crown Court.

U.S. and British authorities undertook a major investigation into LulzSec’s activities. It turned out that Hector Xavier Monsegur, who was known as "Sabu" online, was both a leader of the group and an informant to U.S. authorities on the group’s activities, The Los Angeles Times reports.

The FBI persuaded him to turn into an informant after he was detained by authorities, The Guardian added.

The U.K. government’s inquiry into the group was considered successful. Each of the hackers arrested and belonging to LulzSec or Anonymous was convicted or entered a guilty plea. Four Anonymous hackers were sentenced in January, The Guardian added.

Cleary pleaded guilty to additional charges of constructing a botnet, making the botnet available to others, hacking into computers operated by the U.S. Air Force, and performing a DDoS (distributed denial of service) attack against DreamHost.

Also, Scotland Yard claimed that Davis acted as a spokesman for LulzSec, TechZone360 reported.

Meanwhile, U.S. prosecutors are investigating or prosecuting several suspects in connection with the hacking spree, including Reuters reporter Matthew Keys, who allegedly conspired with Anonymous members to hack into the Tribune Company’s website.

But his attorneys claim he was infiltrating the group for a news story.