The threat, many details of which were never made public, appeared to recede after Azerbaijani authorities rounded up nearly two dozen people in waves of arrests early this year. Precisely who ordered the hits, and why, was never conclusively determined. But U.S. and Middle Eastern officials now see the attempts as part of a broader campaign by Iran-linked operatives to kill foreign diplomats in at least seven countries over a span of 13 months. The targets have included two Saudi officials, a half-dozen Israelis and — in the Azerbaijan case — several Americans, the officials say.

In recent weeks, investigators working in four countries have amassed new evidence tying the disparate assassination attempts to one another and linking all of them to either Iran-backed Hezbollah militants or operatives based inside Iran, according to U.S. and Middle Eastern security officials. An official report last month summarizing the evidence cited phone records, forensic tests, coordinated travel arrangements and even cellphone SIM cards purchased in Iran and used by several of the would-be assailants, said two officials who have seen the six-page document.

Strikingly, the officials noted, the attempts halted abruptly in early spring, at a time when Iran began to shift its tone after weeks of bellicose anti-Western rhetoric and threats to shut down vital shipping lanes. In March, Iranian officials formally accepted a proposal to resume negotiations with six world powers on proposals to curb its nuclear program.

“There appears to have been a deliberate attempt to calm things down ahead of the talks,” said a Western diplomat briefed on the assassination plots, who spoke on the condition of anonymity because of the sensitivity of the intelligence. “What happens if the talks fail — that’s anyone’s guess.”