Jeff Vespa / WireImage 1 / 8
Jeff Vespa / WireImage 1 / 8

By Kat Giantis

A week after Justin Timberlake and Jessica Biel announced they were "mutually" pulling the plug on their four-year relationship, speculation continues over what, exactly, caused their bust-up.

In recent days, blame has been placed on everything from Justin's alleged two-timing to his supposed infatuation with Mila Kunis to a rumored fight earlier this month during a ski vacation.

Now, Us Weekly and People magazine weigh in with two different takes on what led them to splitsville.

Timberlake "had been aggressively trying to end things" since the Golden Globes in January, one of his pals tattles to Us. "He hasn't been happy with Jessica for close to two years."

That timeline tracks to the start of repeatedly denied rift rumors ("Justin Says, 'It's Over!'" Us declared in 2009).

"You'd be surprised at how soon the love was lost on this one," JT's friend callously adds. "Justin said he was miserable."

No word on why the crooner-turned-actor didn't just give the romance the Band-Aid treatment: One painful yank and it's done. But the mag says he eventually came clean about his amour apathy after he hit the big 3-0 on Jan. 31.

"He just said he didn't want to marry her and it didn't feel right," relays a source, who says the actress didn't see it coming. "It just stung her in the heart."

RELATED: Justin 'cozy' with Mila at SAG Awards?

Earlier this week, the London Daily Mail claimed that problems were evident at a party Jessica tossed to celebrate Justin's milestone.

During the soiree, she reportedly made a teary-eyed toast about her love for her beau and what he meant to her. His purported response: "Yeah, b-tches!"

People mag, however, has a different take on the commitment chatter, insisting that the nuptial talk had nothing to do with their decision to part ways.

"The topic of marriage wasn't the catalyst or final straw that broke up the relationship," says an insider.

Still, if Us' sequence of events is accurate, Timberlake confessed his ostensible commitment-phobia just as a report surfaced claiming that Biel had been spied checking out engagement rings at jeweler-to-the-stars Neil Lane.

But a Jessica confidant maintains to People that the bauble browsing never happened: "There was no ring shopping. She wasn't even thinking about marriage. That didn't play a part in their breakup at all."

So what did?

According to Us, during his heart-to-heart with Biel, Timberlake sidestepped rumors that his eye had wandered (at times in the apparent direction of Olivia Munn, who is rumored to have received a text from Justin a few months back that read, "My relationship is basically over").

"He didn't admit the cheating, even though we all knew it," snipes a snitch.

RELATED: Olivia talks to Wonderwall about the Justin rumors

But People's source paints a more sympathetic picture of Justin's feelings, saying he is "upset" over the breakup, which came just weeks after the pair got their PDA on while hitting the post-Oscar party circuit.

"They always spent vacations together. They always spent down time together. He was invested in her, in her friends and family," shares the spy. "He was invested in their relationship on a lot of different levels. After four years it's just not easy to walk away from it."

Read more Hot Gossip