Citi Field became an oversized interrogation room Friday, when interviews began concerning alleged misuses of the Injured list this season.

Major League Baseball has started its Investigation and talked with Mets employees at the team’s Queens home.

When the Mets initially were alerted to the investigation, they were told the alleged injured list manipulation would be the focus.

On Day 1, the talks indeed surrounded IL use.

The Mets were informed by MLB that owner Steve Cohen is not the target of the investigation, a source told The Post.

Of course, the investigation has just launched, and it is possible it could take investigators in unexpected directions.

Billy Eppler resigned as Mets general manager on Thursday, after he was alerted MLB was opening its investigation into his team’s potential abuses of the IL.

A source said the league had been tipped off by an anonymous letter that prompted the probe.

The scope of the letter has not been made public.

Eppler, who previously had been expected to stay on as GM beneath new president of baseball operations David Stearns, plans to fully cooperate.

At the center of the probe is a potential competitive advantage: a fairly common scheme that enables teams to retain players who might be lost otherwise and enables players to secure money that might not have been in reach.

The so-called “phantom injured list” is common enough to have a name.

A team with a full roster could falsify an injury for a healthy player who is out of minor league options rather than risk losing that player, who would become available to other teams if he is exposed on waivers.

The healthy player could then be activated once there is room on the roster, space created by another player getting injured or another player being optioned to the minors.

The tactic allows players on the fringe of a roster to continue receiving major league paychecks and service time while on the IL.

Eppler resigning in the wake of an investigation that seems more everyday than eye-opening has invited skepticism across the league.

“There must be more to this,” one rival executive said.

“Is that really the reason?” another rival exec offered.

In his statement, which preceded the news of the investigation, Eppler said he wanted Stearns to have “a clean slate.”

It is unclear whether the Mets will try to hire Eppler’s replacement this offseason.

If Stearns chooses to fill the spot, a name to watch will be former Phillies GM Matt Klentak, who currently is a special assistant in the Brewers’ front office.

He and Stearns worked together at MLB headquarters in Manhattan.