Marsh, a federal narcotics agent with 29 years' experience, was stationed at Denver. Hereford had been engaged as a 'special employee' of the Bureau of Narcotics at Denver for about six months, and from time to time gave information to Marsh regarding violations of the narcotic laws, for which Hereford was paid small sums of money, and that Marsh had always found the information given by Hereford to be accurate and reliable. On September 3, 1956, Hereford told Marsh that D 'was peddling narcotics to several addicts' in that city. Four days later, Hereford told Marsh 'that D had gone to Chicago the day before [September 6] by train [and] that he was going to bring back three ounces of heroin [and] that he would return to Denver on the morning of the 8th of September or the morning of the 9th of September by train. Hereford gave Marsh a detailed physical description of D and of the clothing he was wearing. Hereford said that D would be carrying 'a tan zipper bag,' and that he habitually 'walked real fast.' On the morning of September 9, police saw a person, having the exact physical attributes and wearing the precise clothing described by Hereford getting off from an incoming Chicago train and who began to start walking 'fast' toward the exit. D was carrying a tan zipper bag in his right hand and the left was thrust in his raincoat pocket. The police officer overtook, stopped and arrested D. They found two 'envelopes containing heroin' clutched in his left hand in his raincoat pocket, and found the syringe in the tan zipper bag. D was arrested. Hereford died four days after the arrest and therefore did not testify. D moved to suppress the evidence from the search. D contends (1) that the information given by Hereford to Marsh was 'hearsay' and, because hearsay is not legally competent evidence in a criminal trial, could not legally have been considered, but should have been put out of mind, by Marsh in assessing whether he had 'probable cause' and 'reasonable grounds' to arrest petitioner without a warrant, and (2) that, even if hearsay could lawfully have been considered, Marsh's information should be held insufficient to show 'probable cause' and 'reasonable grounds' to believe that petitioner had violated or was violating the narcotic laws and to justify his arrest without a warrant. D was convicted. The Appeals Court affirmed and the Supreme Court granted certiorari.