The U.S. Secret Service's acting director said on Friday that local police in Pennsylvania warned that there was a man with a gun on a roof before the July 13 attempted assassination of Donald Trump, but the message did not reach its agents on time.

Local authorities and Secret Service agents were using different communications channels, which prevented the warning from getting through before a 20-year-old assailant opened fire on the Republican presidential candidate, Secret Service Acting Director Ronald Rowe told reporters.

"In the final 30 seconds — which has been the focus of what happened before the assailant opened fire — there (were) clearly radio transmissions that may have happened on that local radio net that we did not have," Rowe said.