Washington (CNN) -- Oil giant BP and contractor Halliburton knew of potential flaws in the cement slurry used to reinforce the oil well below the Deepwater Horizon rig before it exploded in April, according to a letter Thursday from the lead investigator for a federal probe of the Gulf oil disaster.

CNN: STORY HIGHLIGHTS

If a stable column of mud has been set up in the well, keeping the oil at bay through its sheer weight, the next step would be to set cement in the well and seal it off permanently.

ECONOMIST: Nerves are fraying a month after the Gulf of Mexico oil spill

As in most oil states, Iraq's idea is to channel the crude oil wealth into the creation of refineries, steel mills, agriculture, cement factories, as well as sundry infrastructural programs--schools, hospitals, roads and the like.

FORBES: Iraq: The Oil, C'est Moi

The civil lawsuit was brought against the UK oil firm - as well as other companies involved in the accident, including rig-owner Transocean and cement-provider Halliburton - by the US Justice of Department and the US states affected by the oil slick.

BBC: BP: Scope of oil disaster internal probe 'had limitations'