The victim, 21-year-old Oury Jalloh, set a mattress on fire with a cigarette lighter in 2005 while his hands and feet were shackled to the bed. State prosecutors said he could have been saved if police had acted fast enough to get him out of the cell.
A 46-year-old policeman was charged with bodily harm with fatal consequences and his 44-year-old colleague with manslaughter. The accused said they tied Jalloh's hands and feet because he had been violent after being arrested for drunkenly harassing women.
"Despite the intensive efforts of all parties to this trial, we did not have the chance to clear everything up in what could be called a manner in accordance with the law," presiding Judge Manfred Steinhoff said in criticism clearly aimed at the local police.
The case sparked outrage in Germany four years ago, and those attending the trial protested the acquittal by scuffling with police in the courtroom.
Some stormed towards the judge to denounce him as a liar and protestors gathered outside chanted "this ruling is an outrage" and "human rights don't matter anymore in Germany."