A private laboratory has been suspended from processing swabs after the false negatives.
Some 150 students were participating in the cleanup along the banks of the Cileueur when 21 of them slipped into the water.
The early investigation has revealed “a potential motivation linked to Islamist extremism," UK officials said.
Buckingham Palace had no comment or clarification on the queen's remarks.
The incident took place as the Conservative lawmaker was meeting constituents in Essex.
Lava rolling toward the Atlantic Ocean forced the evacuation of another 300 people on La Palma late Thursday, bringing the total number to 7,000.
Ibrahim Rotich was detained after crashing his vehicle during a car chase with police.
"If we’re not careful we’re robbing from our children’s future through what we do now," the Duke of Cambridge said.
The two-time world championship bronze medalist was discovered dead at her home in Iten in western Kenya with stab wounds, said police.
The 4.5-magnitude earthquake was the strongest recorded temblor since volcanic eruptions began on La Palma 26 days ago.