What started out as a cyber attack on Hollywood studio Sony Pictures has snowballed into a national security issue for the United States.

The Federal Bureau of Investigation (FBI), which is leading an investigation into the cyber attack, has warned theatres and other businesses associated with Sony Pictures' film The Interview that they could be targeted in cyber attacks.

CNN reported that the US government would soon make an announcement fingering the North Korean government as the one responsible for the "state-sponsored" attack. North Korea has denied its involvement.

A group of hackers who call themselves Guardians of Peace (GOP) obtained massive troves of data stolen from Sony servers, and leaked sensitive information like salaries, salacious e-mail arguments, and unreleased movies. The group on Dec 16 warned people to stay away from cinemas showing The Interview, a parody film which depicts a fictional CIA plot to kill North Korea's leader Kim Jong Un.

Sony Pictures has since pulled the film from its planned Dec 25 release after top US movie theatre chains cancelled plans to show it. Here is a timeline of events since the movie trailer of The Interview was released six months ago.

June 2014

Soon after the release of the trailer for "The Interview" in June, the North Korean Foreign Ministry responded with a terse statement calling the film "an act of war" and declaring its maker a "gangster filmmaker". It sent a letter to UN Secretary-General Ban Ki Moon.

Nov 24

Hackers seized control of the company's computer system. Employees found the visual of a skull staring at them when they logged into their computers. It was accompanied by an even more disturbing message that said: "We've already warned you, and this is just a beginning. We continue till our request be met. We've obtained all your internal data including your secrets and top secrets."

Nov 27

Five Sony movies are leaked onto online file-sharing hubs. Four were yet to be released- Annie, Mr. Turner, Still Alice and To Write Love On Her Arms. Brad Pitt movie Fury, which is already in theatres, is also shared. The movies are widely downloaded.

Nov 28

First reports pinning the blame on North Korea emerged. Sony showed no signs of stopping the planned release of The Interview.

Nov 28- Dec 1

Sony servers remained down, and employees were unable to access email and voicemail.

Dec 1

Details on pre-bonus salaries of top Sony executives were leaked. Salaries of more than 6,000 current and past Sony employees were also leaked. Sony hired a cyber-security firm to help investigate the attack while FBI confirmed that it has launched its own investigation.

Dec 2

Sony chiefs Michael Lynton and Amy Pascal issued a company-wide alert to employees about the brazen cyber attack. "It is now apparent that a large amount of confidential Sony Pictures Entertainment data has been stolen by the cyber attackers, including personnel information and business documents...While we are not yet sure of the full scope of information that the attackers have or might release, we unfortunately have to ask you to assume that information about you in the possession of the company might be in their possession," they said in the notice.

Dec 3

Further leaks occurred. Information on passport details and visas of cast and crew members who have worked on Sony films, including those of Angelina Jolie and Jonah Hill, were made available online. Film budgets, confidential contracts, and the user names and passwords of Sony executives were leaked.

Dec 5

Hackers claiming to be Guardians of Peace (GOP) issued threats to hurt Sony employees and their families if they do not sign a statement against their company. "Many things beyond imagination will happen at many places of the world. Our agents find themselves act in necessary places. Please sign your name to object the false of the company at the e-mail address below if you don't want to suffer damage. If you don't, not only you but your family will be in danger (sic)," the message said.

Dec 7

North Korea denied involvement in the hacking.

Dec 8

The GOP posted a letter on a file-sharing site, warning Sony to stop "showing the movie of terrorism which can break regional peace and cause the War". The letter denied responsibility for earlier threats against Sony employees and their families.

Dec 9

Email exchanges between studio co-chairman Amy Pascal and producer Scott Rudin were leaked. In one of the emails, Rudin called US actress Angelina Jolie a "minimally talented spoilt brat".

Dec 10

An e-mail exchange between Pascal and Rudin about President Barack Obama's favourite movies, all of them black-themed, was released.

Dec 11

The Interview premiered amid tight security at the Ace Hotel's theatre in downtown Los Angeles. Before the film began, actor Seth Rogen, who starred in the film, thanked Pascal "for having the balls to make this movie".

Dec 13

Hackers released seventh large dump of Sony files and promised a Christmas gift that would put Sony Pictures into the worst state.

Dec 14

An early version of the script for Spectre, the next James Bond film, was leaked. MGM and Danjaq, which own the rights to the script, said they would take all necessary steps to protect their rights.

Dec 16

Hackers warned people to stay away from cinemas showing The Interview, and reminded movie-goers of the Sept 11, 2001 hijacked plane attacks on the US.

Dec 17

Sony Pictures cancelled the release of The Interview.

Dec 25

After coming under fire from President Barack Obama and from free speech advocates, Sony Pictures released The Interview online for US viewers.

Sources: Deadline.com, Reuters, BBC