Published: May 12, 2010 11:39 AM EDT

ON THE GULF OF MEXICO (AP) - A second, smaller oil containment box was lowered into the sea near the blown-out well in the Gulf of Mexico.

The box was being slowly submerged to the seabed Tuesday. But it won't be placed over the spewing well right away. BP spokesman Bill Salvin says engineers want to make sure everything is configured correctly and avoid the same buildup of ice crystals that stymied their first attempt at using a larger box that was about 100 tons.

This box will be connected to a ship on the surface by a pipe-within-a-pipe when it's lowered. Crews plan to pump in heated water and methanol so ice won't build up.

Salvin said undersea robots will position the box over the gusher by Thursday.

More than 4 million gallons of oil have spewed from the well since a drilling rig exploded April 20.