A new study challenges claims from the administration of President Barack Obama that Colombia is making important strides in bringing to justice killers of labor activists and so deserves U.S. congressional approval of a long-stalled free trade pact. The Human Rights Watch study found "virtually no progress" in getting convictions for killings [of trade unionists] that have occurred in the past 4 1/2 years. It counted just six convictions obtained by a special prosecutions unit from 195 slayings between January 2007 and May 2011…"A major reason for this ongoing violence has been the chronic lack of accountability for cases of anti-union violence," Human Rights Watch said in a letter sent Thursday to Colombian Chief Prosecutor Viviane Morales that details the study's findings.