Complex sentence: At the time of the killings, the state of Mississippi made little effort to prosecute the perpetrators, but the FBI, under the pro-civil-rights President Lyndon Johnson and Attorney General Robert F. Kennedy, directed a vigorous investigation.
Split into simpler sentences: At the time of the killings, the state of Mississippi made little effort to prosecute the perpetrators. The FBI, under the pro-civil-rights President Lyndon Johnson and Attorney General Robert F. Kennedy, directed a vigorous investigation.