I got this guide from PS3 Trophies, but I've made some adjustments to make it more accurate. I got the achievement yesterday, but somehow TA.com is not updating my cheevos.
Ask marcus all questions at the start. After asking him all the questions, look to his left by a yield sign and you will see a yellow hardhat to collect. Next, click on the garden in the middle, click on the footprints near the body in the dirt and take a picture. Zoom back and click up at the top of the screen to look up at a balcony. Open your PDA and head up to balcony.
Scroll right and click on ladder, pick up the cable from underneath it. Click on where the victim fell from and the bar where the tape is waving in the wind, spray the blood with the fluids spray. Later, click on the top of the poll and collect a finger print. Go back to lobby and talk to Marcus to get his footprint and dna sample then head to the morgue.
Talk to Dr. Robbins, and then click on the victim. You will have to take two photographs of his neck, then ask the doc some additional questions. Head off to the lab.
Open your PDA, evidence folder and select suicide note. There is a fingerprint in the around the middle of the note (check both sides). Use the fingerprint spray to collect it. Next, select the power cord and get the fingerprint off of where you plug a cord into it.
Go to the database computer, select the fingerprint from railing and victims' fingerprint - it will match with the one at the bottom right. Choose fingerprint from suicide note and hit prepare search. It will match the bottom left side of your only result.
Go to Brass’ office and talk to the FBI agent. After that, ask Brass for a warrant for Zachary Lynch.
Go to casino balcony and click on metal gate, go in and talk to Zachary, ask him all questions. Click on computer and then onto CPU to collect it into evidence; next click on the keyboard and swab the blood on it. Then, in the top right corner of the keyboard, there is a fingerprint to collect.
Ask zachary more questions and head back to lab. Use the database computer, computer analysis and select desktop computer form construction office. Then, go into fingerprints and select the one off the keyboard; prepare search and click on 5 different spots then search, it will match the top right square.
Go to substance analysis and compare blood on keyboard to victims' blood, then compare blood from rail to victims' blood. Go over to the microscope and select the power cord compare it to the larger of the two ligature mark photo. Amplify it 60 times, and adjust focus.
Go to Brass office and ask for a warrant for Zachary, choose bloody fingerprint on keyboard. Ask all questions to Zachary and confront him with hard drive evidence. Ask the agent all the questions then return to the hotel lobby.
Click on the flower garden and then the plant in the back left that looks like its dead. Head to the lab and Greg will update you on the plant. Open your PDA, then evidence, and select the plant. Collect the fertilizer at the bottom. Go to your substance analyzer and put in the substance, choose the 2nd, 5th, 6th, and 8th choices. Head to Brass’ office and ask for a list of buyers.
Ask Todd all questions, he will put his feet on table after asking for his shoes; click on them and take a picture, head to the lab and compare his shoe photo with the photo in the dirt from the crime scene. They will match. Head back to the interrogation room and finish asking him questions, confront him with the shoe evidence.
Go to the balcony and click on the rug to remove it and expose the safe, then click on the safe to open it, go inside it and click on the money wrappers to collect a fiber then collect the laptop and the money wrappers, after this head to Marcus’ office. Head back to the lab and analyze the fiber you found. Choose the 2nd, 3rd, 7th, and 8th options. Head into database computer and go to computer analysis choose laptop from safe and search. In Brass’ office, ask for a warrant for Marcus and choose laptop then head back to Marcus office.
In Marcus office, look around his desk then go to the back corner click on his tool belt and take the drain snake out of the bottom right pocket, look around some more and head back to lab, open your PDA and select the drain snake in the evidence folder take a picture of it. Go into the microscope and compare the picture with the photo of the smaller of the two ligature marks on the victims' neck (amplify 60 times). Adjust the focus and you will have a match.
Open your PDA and bring up drain snake. Click on the top to collect a white fiber; go to the substance analysis and choose the 2nd, 3rd, 7th , and 8th options. Go to Brass’ office and ask for an arrest warrant for Marcus based on drain snake. Ask Marcus all questions. After you get his gloves and DNA (the DNA will be available only after you ask him all the available questions, please be careful), head back to lab, open your PDA and get blood off Marcus’ gloves. Then, collect the epithelial tissue from inside the glove and you should unlock "scavenger hunt".
From this point on, I suggest you create a save back-up. Some of my achievements for completing the cases didn't unlock at first, so I had to load the game and redo the last steps of the case.