Attending ex-girlfriend's wedding, arresting the groom's officer on the spot

"The body was lying face down, with its head facing west and its feet facing east. Its upper body was embedded in the weeds at the bottom of the ditch, while its lower body was exposed on the water's surface. Its clothes were soaked by the water and clung tightly to its body." Yang Lin took photos of the body in its original state with his camera, recording it from multiple angles, including the front, side, and back. Yang Sen recorded the entire process with video equipment.

"First, clear away the weeds and trash around the body, being careful not to touch the body or disturb the evidence." Yang Lin took out tweezers and bamboo strips and carefully parted the weeds around the body. The weeds in the ditch were mainly foxtail grass and barnyard grass, some of which were trampled and flattened, facing the body, presumably formed when the body fell into the ditch. About 30 centimeters west of the body's head, Yang Sen found a metal button: "Relic from Core No. 1, a metal button, about 2 centimeters in diameter, with oxidation marks on the surface, and no obvious markings." He used tweezers to pick up the button and put it into the evidence bag.

After clearing away the surrounding weeds, the two began to focus their investigation on the bottom and walls of the ditch near the body. The bottom of the ditch contained a small amount of silt, and the water was murky. Yang Lin shone his searchlight into the water and discovered several irregular depressions in the silt, which he speculated were formed by the body struggling after falling in. In the silt below the body's right hand, he found a clear fingerprint: "Core Fingerprint No. 1, left on the surface of the silt, with clear lines, preliminarily identified as an index finger fingerprint, with the fingertip pointing towards the body."

Because the fingerprints were left on the moist mud surface, they couldn't be directly developed with powder. Yang Lin took out fluorescent powder and a spray bottle, mixed the fluorescent powder with anhydrous ethanol in a certain ratio to prepare a fluorescent solution, and gently sprayed it onto the fingerprint area. After the solution dried slightly, he turned on a multi-band light source and selected 450nm wavelength blue light for illumination. The fingerprint patterns became clearly visible under the fluorescence. Yang Lin immediately took a photo of the fluorescent fingerprint, then covered the fingerprint with a fingerprint extraction film, gently pressed it, and carefully peeled it off to complete the fingerprint extraction.

On the south side of the ditch, about 20 centimeters from the water surface, Yang Lin found several scratches: "The scratches are longitudinal, ranging from 5 to 8 centimeters in length and about 0.2 centimeters in depth, with irregular edges. It is speculated that they were formed by the scraping of hard objects, possibly left by tools used by the suspect when moving the body, or by hard objects on the clothing when the body fell in." He used a fiber extractor to extract a small amount of green fiber fragments from the scratches, which he labeled as "Core No. 1 Fiber Sample".

The two men extended their investigation 5 meters to the east and west of the drainage ditch. At the bottom of the ditch, 3 meters to the west, they found a broken nylon rope: "Remnant from Core 2, nylon rope, approximately 1.2 meters long and 0.5 centimeters in diameter. One end shows obvious signs of breakage, the other end is worn, and the surface is covered with a small amount of silt." Yang Lin picked up the nylon rope with tweezers and examined it carefully. He discovered several white fibers wrapped around the rope, a different color from the fiber sample from Core 1. He packaged the nylon rope and the wrapped fibers separately and labeled them clearly.

"Check the ditch walls for any signs of climbing." Yang Lin shone his survey light on the ditch walls, which were made of brick and stone with a rough surface and covered in moss in several places. On the ditch wall north of the body, two fresh climbing scratch marks were found: "The scratch marks are about 0.3 centimeters deep, with clear fingertips pointing upwards, suggesting that someone climbed up from inside the ditch." He treated the scratch marks with the same fluorescent solution and extracted two blurry fingerprints, labeled "Core Fingerprint No. 2 and No. 3".

In the core exploration area, the two investigators also collected multiple trace samples from the soil and silt, including silt samples, weed samples, and water samples, for subsequent testing. They also made a preliminary observation and record of the body's clothing: the deceased was wearing a gray long-sleeved T-shirt, dark blue overalls, and black cloth shoes. The clothing showed no obvious damage, but there were tears at the collar of the T-shirt and the hems of the overalls, presumably caused by being snagged by weeds or sharp objects when falling into the ditch. There were no obvious bloodstains or dirt on the clothing, only a large amount of silt and weeds.

核心勘查区的勘查工作极为细致，每一个环节都反复确认，避免遗漏。两人共提取指纹4枚、足迹样本2枚补充中距勘查区、遗留物2件金属纽扣、尼龙绳、纤维样本2份、微量物质样本6份，拍摄照片218张，录制视频45分钟。整个核心勘查过程持续了2小时30分钟，直到中午12时50分才基本结束。

Subsequently, the two entered the "re-examination area" to conduct a second review of the previously explored distant, mid-distance, and core areas, focusing on checking for any missed traces and verifying the location and characteristics of the extracted traces. Next to the tire tracks in the mid-distance exploration area, Yang Lin extracted a small amount of black rubber particles, labeled "Mid-distance Trace Material Sample No. 2"; at the break in the nylon rope in the core exploration area, a small piece of metal residue was found, presumably from the wear and tear of the knot, and was also extracted and sealed.

At 1:10 PM, the on-site investigation was completed. Yang Lin and Yang Sen categorized and organized all the collected trace evidence, filling out a detailed evidence list that noted the time, location, characteristics, and quantity of each piece of evidence, ensuring that every piece was traceable. They then reported the investigation results to Lu Chuan.

"Team Leader Lu, the on-site investigation is complete. We strictly followed the principle of 'from far to near, from outside to inside' and collected a total of 32 pieces of various trace evidence." Yang Lin opened the investigation logbook and reported one by one, "In the outer area, we collected one 41-yard rubber rain boot footprint and one black fiber fragment; in the mid-range area, we collected one sneaker print, one leather shoe print, one rain boot print, one section of tire mark from an electric tricycle or small truck, one bark sample, and two trace substance samples; in the core area, we collected four fingerprints, one metal button, one section of nylon rope, two fiber samples, six trace substance samples, and multiple soil, silt, and water samples."

"What are the key clues we have so far?" Lu Chuan asked. Yang Sen added, "First, four fingerprints. The core fingerprint, number 1, has clear lines and is suitable for comparison. The other three fingerprints are relatively blurry, but can serve as supplementary evidence. Second, five footprints, covering three types: rubber boots, sneakers, and leather shoes. This indicates that at least three people were active at the scene and in the surrounding area. The person wearing the rubber boots has a track record that runs through the outer, middle, and core areas, making them a prime suspect. Third, tire tracks, presumably left by a means of transportation, possibly related to the movement of the body. Fourth, a nylon rope and metal buttons. The nylon rope shows signs of breakage and wear, suggesting it might have been part of a tool used in the crime or for transporting the body." (End of Chapter)