Scientists in the United States are still not sure that the damaged oil well in the Gulf of Mexico has been securely sealed.
The latest operation to seal the BP well involved pumping in mud and cement to create a cap.
No oil has escaped for almost a month. However, tests have indicated a possibility that the seal may not be strong enough.
The leak began after an explosion and fire on the Deepwater Horizon drilling rig exploded on 20 April. The rig, which was owned by Transocean and leased to BP, sank two days later.
The leak took until 15 July to halt.