While laying down 5″ drill pipe the crew proceeded until there were 3 stands in the well. The Company Man stopped the operation and instructed the drill crew to use the cementing unit pump to conduct an integrity test on the cement plug and casing against the BOP top pipe rams. A pre-job safety meeting was conducted by the Senior Toolpusher (STP) for the driller and the drilling crew. After pressure testing, the cement plug / casing showed pressure leakage. The pressure was bled off and the STP went to the Company Man’s office for further instructions. Prior to leaving the rig floor the STP informed the driller not to forget to open up the top rams and to pull out the string. The driller decided to assist the crew in disconnecting and lying down the cementing line, valve and circulation head prior to pulling the final three stands of drill pipe. When starting to pull the last three stands out of the hole the driller felt an over-pull and remembered that top rams were still closed. No remarkable change in Martin Decker reading was observed by the driller since the lower tool joint wasn’t against the top rams due to the spacing out for pressure testing. When the tool joint reached to the top rams, the drill pipe started to stretch and unfortunately, the driller didn’t have enough time to respond and stop the operation especially, since the string had already traveled 6 ft. upwards. The drill string parted and the rest of the string (8 joints of pipe + about 27’of the parted joint of DP) was dropped down into the well.