The Jim Harbaugh era is over after four largely successful years, but the San Francisco 49ers have a deep list of candidates to fill their head coaching vacancy. General manager Trent Baalke immediately began the process of finding the 49ers next head coach, and the wide-ranging list of candidates largely includes current NFL coordinators and one former Super Bowl winner.

It appears Baalke and the organization is certainly considering promoting from within, with current defensive coordinator Vic Fangio already interviewing with his superiors for six hours on Tuesday, according to CBS Sports reporter Jason La Canfora.

A coordinator and assistant in the NFL for nearly 30 years, Fangio is eyeing his first stint as a head coach after helping Harbaugh mold the 49ers into one of the best defensive units in the NFL over the last four years. Despite missing the playoffs largely due to major issues on offense, the 49ers once again fielded a top 10 defense under Fangio’s leadership, ranking fifth against the pass and seventh versus the run.

Other than the length of his interview, Fangio’s chances of landing the gig were likely increased when, according to the San Francisco Chronicle, the 49ers refused to give the Washington Redskins the opportunity to interview him. The Redskins requested permission to speak with both Fangio and current secondary coordinator Ed Donatell, but allowed only the latter to interview for their head coaching vacancy.

The same report indicated that in years past the 49ers have blocked other teams from speaking with Donatell, but now they appear willing to let him go.

Keeping and promoting Fangio seems like the best way for the 49ers to keep the defense running smoothly, and allows them to keep up with defensive force and NFC West rival Seattle, but it does little to answer for the regression of quarterback Colin Kaepernick. In the first year of his mega six-year, $114 million contract, Kaepernick posted a career-worst 86.4 passer rating and 7.0 yards per pass attempt while San Francisco’s passing game plummeted to No. 30 in the league.

Kaepernick’s major slide could also be a reason why the 49ers are meeting with current Denver Broncos offensive coordinator Adam Gase.

Gase, 36, has gradually moved up within the Broncos organization since he joined in 2009 after a one-year stint as an offensive assistant with the 49ers. He specifically helped quarterback Peyton Manning in his return from neck surgery in 2012 and was the main architect of the highest scoring offense in league history in 2013.

However the 49ers have some intense competition for Gase’s services with the Chicago Bears and Atlanta Falcons also interested in him. According to the Denver Post, Gase met with San Francisco and Atlanta Friday, and has a meeting scheduled with Chicago for Saturday.

Another option is Detroit Lions first-year defensive coordinator Teryl Austin, who in his fourth season as an NFL assistant helped Detroit roll out the league’s No. 2 ranked defense and make the postseason.

The Detroit News reported Thursday that both San Francisco and Atlanta have requested permission to speak with Austin, however with the Lions playing Dallas on Wild Card Weekend, he won’t be available to sit down until next week at the earliest.

Austin also told The News that his availability hinges on whether the Lions prove victorious in the postseason for the first time since 1991.

"A lot of it would have to do with if our team here is ready," Austin said. "If I think I need to get our team ready here, then I won't interview, but if I feel I've got our team (ready) and everything we have in here so we can go play a quality game, then I would.”

The list of coordinators up for the job is rather extensive, but former head coaches are also being considered, most notably former Broncos and Redskins coach Mike Shanahan and recently-fired New York Jets coach Rex Ryan.

The Post reported that both Baalke and 49ers owner Jed Yorke were in Denver to interview not only Gase, but also Shanahan. The 62-year-old served as San Francisco’s offensive coordinator from 1992 to 1994, and left to take over Denver, where he won back-to-back Super Bowls.

However, Shanahan’s most recent head coaching job ended in disaster. After four years in Washington he posted a 24-40 record, with only one winning season and one postseason appearance, and ultimately clashed with quarterback Robert Griffin III.

As for Ryan, who was let go by the Jets after six seasons, the New York Post reports he will sit down with the 49ers on Sunday. Ryan will also meet with the Falcons, and even if he doesn’t land a new head coaching job he’s already received interest from ESPN to work as an NFL analyst, according to the New York Daily News.