From left: Evan Gershkovich, Alsu Kurmasheva and Paul Whelan shortly after their release from detention in Russia. (U.S. Government photo/Handout)

Financial news outlet Bloomberg has disciplined several employees who broke an agreement to keep details of a prisoner swap involving the United States, Russia and other countries a secret until the arrangement had been carried out.

Last month, several news outlets — including Bloomberg — learned details of the proposed prisoner swap involving Wall Street Journal correspondent Evan Gershkovich, ex-u.S. Marine Paul Whelan and a number of others who were wrongly held captive in Russia.

Reporters from the news outlets were briefed on the matter by officials at the White House, the U.S. State Department and other government agencies while the prisoner swap was in the planning and delivery stages, according to accounts that emerged once the swap was completed. Gershkovich, Whelan and other Americans held in detention were released last week,...