Restricted Laboratory
From Wikitroid, the Metroid wiki
The Restricted Laboratory is located on the Restricted Zone of the Biologic Space Laboratories research station. It is at the bottom of the Main Deck, in Sector 6 (NOC), and is designed to be detached in case anyone discovered the work inside there. The Galactic Federation designed it to house the captive-bred Metroids.
When Samus discovered it, an alarm was set off, repeating the words "No entry without authorization." Samus left and ran into B.O.X., finishing it off after an earlier encounter in Sector 3. After defeating it, Samus absorbed its X host and regained the Wave Beam ability. With this ability, she was able to break in to the Restricted Lab.
Inside the lab were a Metroid, Alpha Metroid, Gamma Metroid, and Zeta Metroid being kept in large green tubes that resemble the Baby Metroid's container on Ceres Space Colony. While she was looking at them, one of the SA-X found it too and released the Infant Metroids in an attempt to kill them, Metroids being the X's natural enemy. Its plan backfired and the Metroids began to attack the SA-X. A 1-minute countdown began to the detachment of the lab, and Samus was able to escape before the lab detached from the B.S.L. research station and self-destructed, killing the SA-X and what were apparently the last living Metroids (though it is possible there are still Metroids being kept by either the Galactic Federation or Space Pirates somewhere). At least one Metroid escaped and advanced to the Omega Metroid stage.
Incidentally, the only actual Metroids in Metroid Fusion aside from the escapee and Samus, herself, are found in the Restricted Lab, and they do not actually damage Samus, though they do fly around in a frenzy as Samus attempts to escape, potentially becoming obstacles as she Space Jumps up a long shaft to escape. The most likely reason that the Metroid do not attack Samus is because they see her as one of them due to the Metroid DNA in her body (the Omega Metroid is the exception to this, seeing as how it attacks Samus).
