"Regarding cross-era comparisons, it is generally more difficult to find information on extremist-related killings from the 1970s and 1980s, so it may not be meaningful to compare figures from earlier eras with figures from the 1990s or later. The main limitation of cross-movement comparisons is that extremist connections to killings are easier to determine for some movements than for others. For example, white supremacists, who often display many racist and white supremacist tattoos, or who may be documented as white supremacists by gang investigators or corrections officials, are often more easily identifiable. In contrast, it may be more difficult for police or media to identify a suspect’s anti-government extremist associations. This issue comes up more often with non-ideological killings. It is likely that non-ideological murders committed by extremists other than white supremacists are underrepresented in ADL’s data. In addition, because murders behind bars tend to often attract little or no media attention and are typically not publicized by prison officials, incidents of prison-based deadly violence committed by adherents of all extremist movements are under-represented."