For nine years on Fox's hit sci-fi show "The X-Files," FBI agents Fox Mulder and Dana Scully searched for an alien truth that was seemingly hidden in the skies. But as they return to screens this week, the truth might be much closer to home.

Starting Sunday, six new episodes will reunite Mulder, played by David Duchovny, and Scully, portrayed by Gillian Anderson. The pair spent nine seasons investigating cases involving government cover-ups, unidentified flying objects and "monsters" stemming from folklore.

From 1993 to 2002 the series followed Mulder, a dogged believer in extraterrestrial existence and unexplained phenomena, and Scully (Gillian Anderson), a sceptic with a medical degree, who tried to debunk her partner's theories.

The series ended with the two on the run as an alien invasion threatened human existence.

As the pair reunite, the years have taken a toll. Scully is a surgeon, while a depressed Mulder has isolated himself from the world. They are not in a relationship, and neither is in the FBI any more.

The world, meanwhile, has only served up more unexplained phenomena, said series creator Chris Carter.

"Conspiracy has gone mainstream," Carter told Reuters. "We've got a government who admits to spying on us, so we are in a brave new world of sorts."

In the new series, an Internet-based conspiracy theorist piques Mulder and Scully's interest with a young woman who says she has been abducted numerous times. The culprits might not be little green aliens, but rather, a human conspiracy.

"It actually tests Scully's faith in science being able to explain everything as it always has," Carter said. "It lights a fire under Mulder. I think that it not only reignite the series, but the quest of the two characters."

Carter said "The X-Files" could continue as a limited series, pending ratings and Duchovny and Anderson's respective schedules as both have had success post "X-Files."

The "X-Files" finale in 2002 coincided with a shift in people's perceptions of the U.S. government in a post-9/11 world, Carter said.

"The idea that the government was conspiring against us was contradictory to what people wanted to believe, which is that the government could protect us," Carter said.

But in 2016, conspiracy theories still run rife on the Internet and UFOs still perplex people.

"I believe that we are all interested in the supernatural, whether it be faith, or science," Carter said.