▁testim ony ▁is ▁presented ▁by ▁the ▁pro sec ution ▁to ▁establish ▁that ▁the ▁subst ance ▁in ▁evidence ▁was ▁in ▁fact ▁in ▁the ▁possession ▁of ▁the ▁def endant . ▁ ▁An ▁ident ifiable ▁person ▁must ▁always ▁have ▁the ▁physical ▁cust ody ▁of ▁a ▁piece ▁of ▁evidence . ▁In ▁practice , ▁this ▁means ▁that ▁a ▁police ▁officer ▁or ▁detect ive ▁will ▁take ▁charge ▁of ▁a ▁piece ▁of ▁evidence , ▁document ▁its ▁collection , ▁and ▁hand ▁it ▁over ▁to ▁an ▁evidence ▁cl erk ▁for ▁storage ▁in ▁a ▁secure ▁place . ▁These ▁transactions , ▁and ▁every ▁succeed ing ▁transaction ▁between ▁the ▁collection ▁of ▁the ▁evidence ▁and ▁its ▁appearance ▁in ▁court , ▁should ▁be ▁completely ▁documented ▁chron olog ically ▁in ▁order ▁to ▁with stand ▁legal ▁challeng es ▁to ▁the ▁authentic ity ▁of ▁the ▁evidence . ▁Document ation ▁should ▁include ▁the ▁conditions ▁under ▁which ▁the ▁evidence ▁is ▁gathered , ▁the ▁identity ▁of ▁all ▁evidence ▁handlers , ▁duration ▁of ▁evidence ▁cust ody , ▁security ▁conditions ▁while ▁handling ▁or ▁storing ▁the ▁evidence , ▁and ▁the ▁manner ▁in ▁which ▁evidence ▁is ▁transferred ▁to ▁subsequent ▁cust od ians ▁each ▁time ▁a ▁transfer ▁occurs ▁( al ong ▁with ▁the ▁sign atures ▁of ▁persons ▁involved ▁at ▁each ▁step ). ▁ ▁M aint aining ▁a ▁chain ▁of ▁cust ody ▁is ▁essential ▁for ▁the ▁for ens ic ▁scient ist ▁that ▁is ▁working ▁on ▁a ▁specific ▁criminal ▁case . ▁The ▁documentation ▁of ▁evidence ▁is ▁key ▁for ▁maintain ing ▁a ▁chain ▁of ▁cust ody ▁because ▁everything ▁that ▁is ▁done ▁to ▁the ▁piece ▁of ▁evidence ▁must ▁be ▁listed ▁and ▁who