At least one motorist was killed and 20 more injured, in an explosive pileup on the Long Island Expressway this afternoon, Suffolk County authorities said.

An 18-wheel truck crashed and caught fire near exit 68, in eastbound lanes in the hamlet of Shirley at about 2:50 p.m., cops said.

All lanes, between exits 65 and 69 in both directions, were closed down.

Officials said about 40 cars were involved in the massive, chain-reaction pileup.

Authorities were optimistic that none of the 20 injured had life-threatening wounds.

Additional reporting by David K. Li