Final answer:
Ida B. Wells focused her investigations and writings on racial discrimination and violence against African Americans, notably documenting and exposing the horrors of lynching to challenge and resist racial oppression.
Step-by-step explanation:
The main focus of Ida B. Wells' investigations and writings after 1892 is best described as racial discrimination and violence against African Americans. Wells dedicated herself to documenting the lynching of African Americans and was a vehement critic of its use as a tool of racial oppression.
Her significant work includes the publication of 'Southern Horrors: Lynch Law in All Its Phases,' which exposed the frequency and cruelty of lynching, essentially galvanizing the anti-lynching movement.
Moreover, Wells helped establish the NAACP and actively pushed for the enfranchisement and betterment of African Americans and women's rights, demonstrating her broad interest in social justice issues.