How did researchers discover that vitamin B was actually a group of different vitamins?

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Several translucent, oval-shaped vitamin E gel capsules are scattered on a white surface near a partially visible bottle. — vitamin Vitamin E in gel-cap form.

In 1913, American researcher Elmer McCollum categorized vitamins into two groups: "fat-soluble A" and "water-soluble B." Initially, vitamin B was believed to be a single entity.1

However, as research progressed, scientists discovered that the water-soluble growth factor labeled as vitamin B was not a single substance but a complex of at least two distinct factors. Researchers determined this by observing that only one of these factors was capable of preventing polyneuritis in pigeons. The factor required by pigeons was designated as vitamin B $_{1}$ (later identified as thiamin), while the other factor, which was found to be essential for rats, was designated as vitamin B $_{2}$ (later identified as riboflavin).1