Blood and soil testing confirmed the health consequences. Then we tracked individual shipments to the United States.

We followed the supply chain for batteries used in millions of U.S. cars to villages in Nigeria where people are being poisoned by lead. Peter S. Goodman, who covers economics and geopolitics for The Times, describes our yearlong investigation.

Post-election violence has tarnished the country’s reputation for stability, and the crackdown may have backfired on the government, as officials in Washington call for a re-examination of U.S. ties.