The Justice Department is examining Gustavo Petro for possible ties to drug traffickers. U.S. officials have told him that for now he does not face criminal charges arising from the probes.

© Federico Rios for The New York Times

A baffling overdose death took investigators to the frontier of ultra-potent synthetic drugs. The clues were hauntingly familiar.

Sin Tung Chan was a member of a prominent hometown association in the city, one of hundreds of social clubs that often maintain close ties with the Chinese government.