The Obama administration denied any role in Wednesday’s killing of an Iranian nuclear scientist, the latest in a series of events that have exacerbated tensions with Iran.

The assassination of Mostafa Ahmadi Roshan was the latest in a year that has already seen new U.S. economic sanctions, threats to bar American ships from the Persian Gulf, an Iranian death sentence to a jailed U.S. citizen and an escalation in Tehran’s uranium enrichment program.