In most cases the cells are live but the medium get turbid, which leads to cell death, as i observed. The medium I use is DMEM-F12 with 15% FBS and 1% Strep-Pen as supplement.
Thanks @ sabine strehl for your answer. In fact, I am looking for non-infectious causes, especially the cells behavior or medium when it is 2-3 weeks old.
If you work with primary culture of BMSCs and the medium get turbid, it is probably bacterial contamination or hemolysis. Try a gram-test to check contamination, asses the pH to ensure that it is in a suitable range, and observe if there is no red cells.