The scientific journal that published a controversial Facebook experiment on mood manipulation said Thursday it was concerned that the company did not follow scientific ethics and principles of informed consent.

While it stopped short of retracting the study, the Proceedings of the National Academy of Sciences said it typically publishes experiments that have allowed subjects to opt out of research.

Facebook appeared to have been exempt from this rule because all users agree to a policy on data use when they open an account, constituting informed consent for research, according to PNAS.

"Based on the information provided by the authors, PNAS editors deemed it appropriate to publish the paper," said a statement by editor-in-chief Inder Verma.

"It is nevertheless a matter of concern that the collection of the data by Facebook may have involved practices that were not fully consistent with the principles of obtaining informed consent and allowing participants to opt out."

The journal explained that the US government protects those who participate in research by establishing best practices that scientists obtain informed consent and allowing subjects to opt out, a policy known as the Common Rule.

It said that Cornell University reviewers determined ahead of publication that Facebook's experiment did not fall under the government's human research protection program because it was conducted for internal purposes.

"Adherence to the Common Rule is PNAS policy, but as a private company, Facebook was under no obligation to conform to the provisions of the Common Rule when it collected the data used by the authors, and the Common Rule does not preclude their use of the data," Verma said.

- 'Messed with minds ' -

PNAS's statement followed a formal complaint filed by privacy activists to US regulators seeking an urgent investigation.

In its complaint to the US Federal Trade Commission, the Electronic Privacy Information Center said the study -- which has been widely criticized online -- deceived consumers and violated an agreement on privacy settings.

"The company purposefully messed with people's minds," the complaint said.

It said that Facebook's manipulation of users' news feeds to elicit positive and negative emotional responses also "failed to follow standard ethical protocols for human subject research."

The complaint asked the US regulatory agency to investigate, to order a halt to any similar practices and to require Facebook to make public its algorithm used for the news feed.

An FTC spokesman declined to comment on the complaint.

The privacy group, which joined other groups in complaints in 2009 and 2010 that led to Facebook's 20-year agreement with the FTC on privacy, said that the huge social network did not get users' permission to conduct the study and failed to notify participants that their data would be disclosed to researchers.

The complaint came a day after British authorities announced an investigation over the Facebook experiment.

Facebook's chief operating officer Sheryl Sandberg apologized for communicating "terribly" about the research and maintained that "we take privacy at Facebook really seriously."

The social network clandestinely altered the emotional content of feeds of nearly 700,000 users, giving some sadder news and others happier news in the 2012 study aimed at better understanding "emotional contagion."

The research, published last month, prompted online anger and questions about the ethics of the study, and put the world's most popular networking site on the defensive.

A 2012 settlement with the FTC prohibits Facebook from misrepresenting the privacy or security of its data.