On Jul 23rd 2015 the NTSB released their final report concluding the probable cause of the accident was:

The captain's attempt to recover from an unstabilized approach by transferring airplane control at low altitude instead of performing a go-around. Contributing to the accident was the captain's failure to comply with standard operating procedures.

In the press release of Jul 23rd 2015 the NTSB argued: "NTSB found that the first officer was conducting the approach, and the captain took control away from the first officer, but not until the plane was 27 feet above the ground. This late transfer of control from the first officer to the captain resulted in neither pilot being able to effectively monitor the airplane’s altitude and pitch attitude. According to the Southwest Airlines Flight Operations Manual, the captain should have called for a go-around well before this point in the approach instead of trying to salvage the landing. For example, Southwest’s stabilized approach criteria require an immediate go-around if the airplane flaps are not in the final landing configuration by 1,000 feet above the ground. In this case, the flaps were not correctly set until the airplane was 500 feet above the ground."

The captain (49, ATPL, 12,522 hours total, 7,907 hours on type) was pilot monitoring, the first officer (44, ATPL, 5,200 hours total, 1,100 hours on type) was pilot flying.