SAUGUS – State Troopers arrested a Saugus man and charged him with possession of child pornography Wednesday morning, after a five-month-long investigation.Michael A. Johnson, 50, of 12 Sumner St., was charged with distributing material of a child in the nude and possession of child pornography and was ordered held in lieu of $500 cash bail by Lynn District Court Judge Albert Conlon on Thursday.According to the State Police arrest report, troopers began an online investigation in May to identify people possessing and sharing child pornography. The report states police were given a tip from a police department in Arizona that a computer with the IP address matching Johnson’s was sharing suspected child pornography.The report states that Trooper Michael Murphy sent a “browse request” to the suspected computer and, in turn, the host computer allegedly sent a list of shared files.”Trooper Murphy viewed this list of shared files from the host computer and determined it to contain approximately 450 files,” said the report.According to the report, police downloaded four files from the computer, and according to Trooper James Dowling, “based on my training and experience (I) believe these files to be child pornography in violation of Massachusetts General Law.”The police report said the files included a video with females ranging from ages 3 to 13 engaging in sexual intercourse and photos of what the documents called “prepubescent” and “juvenile” girls having sex with men.State Troopers obtained a search warrant from Lynn District Court on Oct. 17 and according to the report, arrested Johnson two days later.Police interviewed Johnson at his home and according to the report, “He stated he looked at child pornography and he preferred young girls. He stated he entered search terms ‘young teens’ and ‘schoolgirl.'”The report also states that after police conducted a forensic preview of the computer, “Trooper Finn also discovered, in his opinion, several notable files of children either in a state of nudity or in a state of sexual conduct ? ” said the report.A woman identifying herself as Johnson’s mother declined to comment to the Item in a phone call Thursday night.Matt Tempesta can be reached at [email protected]. You can follow him on Twitter @MattTempesta.