How GNSS Data Contributed to United States v. Dzhokhar Tsarnaev
When Sgt. Timothy Dowd arrived in Watertown, Mass., four days after the 2013 Boston Marathon bombing, the residential neighborhood was swarming with police. Everyone was looking for Dzhokhar Tsarnaev. Just hours before, an early morning firefight had taken place on the streets of Watertown with Dzhokhar and Tamerlan Tsarnaev, the brothers wanted for the bombing and the subsequent murder of MIT patrol officer Sean Collier. After his brother was killed, Dzhokhar fled the scene on foot.
Crime scene technicians from the Massachusetts State Police had worked through the night locating and marking evidence. Now it was up to Dowd, a 14-year veteran of the agency’s Collision Analysis & Reconstruction Section (C.A.R.S.), to document the extensive scene. Just five months prior to this incident, the vehicular homicide unit had transitioned from total station to Leica Geosystems GNSS technology to meet quick-clearance goals. “At the time of this incident, the C.A.R.S. Section only had one GPS unit, and that was assigned to me,” Dowd explained. “That’s why we were deployed.”
Arrival on the Scene
It was shortly before noon that Dowd arrived at the scene with C.A.R.S. Commander Lt. Andrew Klane, and two troopers. “Lt. Klane was the liaison with fellow law enforcement,” Dowd said. “Sgt. O’Hara and Trooper George drew field sketches and took field notes containing all the vehicles and houses that were in the scene. That information would allow me to identify all of the items we mapped and create the diagrams later on.”
Dowd and his team coordinated with the various crime scene techs. The enormous scene had been divided into four sections, and several teams were marking the evidence. “We would get with the team leader of each crime scene,” Dowd said, “and they would essentially identify the evidence they thought was important to document.”
Documenting a challenging scene
It was a dangerous situation. “We didn’t know where he was,” Dowd said. “He could have been hiding in any of the houses in that neighborhood, and we were outside mapping. We were mainly focused on mapping the exorbitant amount of evidence. Honestly, our guard was down a little bit, so that part of it was very stressful, and the chaos that was going on there made it more stressful.” Dowd recounts the moment when Dzhokhar Tsarnaev was discovered in a neighborhood adjacent to where he was shooting points. “All of a sudden, the crime scene technicians who were assisting us got a call, jumped in a car and took off and sort of left us standing in the middle the road,” he said. “Then 30 seconds later, we heard what sounded like hundreds of gunshots. We later learned that a shootout ensued when the brother was found in a boat in a backyard.”
As Dowd and his team set to work, they faced an ever increasing number of challenges—not the least of which was the fact that the remaining brother was at large and highly dangerous. SWAT teams and several hundred police officers were searching the neighborhood. “It was an unusual environment,” Dowd said. “We had an ongoing manhunt, but we also had several crime scenes to document within that active scene.”
Beside safety, Dowd’s No. 1 challenge was the enormity of the scene. Evidence was strewn over the streets and yards of several neighborhoods. “There was more evidence to collect than I’ve ever seen,” Dowd said.
“When these brothers got into the shootout, they threw a pressure cooker bomb at the police, which exploded and went through a car door. They threw pipe bombs at the police, sending fragments into houses and cars. There were rounds shot into all of the houses that we had to somehow document. We did that by taking points below the location where the rounds went into the houses and, later, used photos to assist when creating the diagrams.”
GNSS technology overcomes challenges
The Leica Geosystems GNSS solution was the right mapping technology for the job. “The equipment worked perfectly,” Dowd said. “In fact, without this piece of equipment, it would’ve been very difficult to document all the evidence that was located throughout the scenes.”
With other measurement technology, Dowd would need a clear line of sight, but this residential neighborhood was full of obstructions—some of them moving. In addition to bushes, houses, cars parked along the street, etc., there were hundreds of law enforcement officers walking through the crime scenes as he worked. “I can’t even imagine having to do it by total station, which is what we had previously used,” he said. “To go around these neighborhoods with a total station, we’d have to traverse many times, which means we would have to pick up the instrument and move it from location to location in order to see all the points. With the GPS we just needed to see the sky.”
The GNSS system eliminated the need to traverse. “With the GPS, we could be working in the middle the street and then walk around into the backyard of a house and shoot a point. There was no challenge to doing that because we weren’t shooting back to any sort of set station, so none of the points have to coordinate with each other. Then, using a georeferenced orthoimage, the points appear on the image exactly where they are.”
A few times, foliage or man-made structures obstructed the satellite signal. “In that case, we set a couple reference points, shot those points with the GPS and triangulated to the piece of evidence if it were under tree or under the porch of the house, etc.,” he said. “But otherwise, we had no issues with the satellite visibility at the scene.”
How the data was used at trial
The Leica Geosystems GNSS mapping technology also allowed Dowd to work efficiently and accurately. “Essentially, all we had to do is place the instrument on the piece of evidence and put in a code—for example, if it was a cartridge casing, we have a designated code for that. We input the code, hit the measure button, and literally, in three or four seconds, it recorded the position of that piece of evidence.” And because Massachusetts Department of Transportation operates its own CORS network, those measurements were computed with extremely high accuracy. “MassDOT has 18 base stations set throughout the state,” Dowd said. “The CORS network allows for accuracy of 1 to 3 centimeters.”
In only eight hours, Dowd and his team shot every piece of evidence the crime scene techs identified from all four crime scenes. “That totaled 448 points,” Dowd said. “Those points of evidence included weapons, bomb fragments, pressure-cooker fragments, spent cartridge casings, blood, civilian and police vehicles that were involved, and bodies—the location of Tamerlin Tsarnaev after he was struck by the vehicle operated by his brother Dzhokhar and the location of a police officer who was shot at during the Watertown event. The officer’s vehicle and the officer’s location were measured. The location where Dzhokhar’s vehicle was ditched was also mapped. Rounds were fired at another vehicle a couple neighborhoods away, and those need to be documented, as well.”
Working in AutoCAD Civil 3D, Dowd created a total of 14 scene diagrams, which he turned over to the U.S. Attorney’s Office for the trial. In addition to Watertown, the diagrams included the Massachusetts Institute of Technology campus, which was documented with a Leica Nova MS50 MultiStation and a Leica ScanStation 3D laser scanner.
The diagrams provided different views and versions. “For trial, the U.S. attorneys wanted versions with just the ballistic evidence. They wanted a version with just the explosive evidence. They wanted a version without any evidence. They wanted versions with different zooms. They wanted versions of MIT without the cruiser, with the cruiser, etc.,” he said. “And they wanted all the evidence linked to the photographs that would be displayed in court.”
Dowd’s diagrams and the data collected with the Leica Geosystems survey-grade mapping technologies were integral to the prosecution’s case. “It really helped guide the whole trial for the U.S. attorney,” Dowd said. “They used the diagrams to display what happened at the scenes, and when they wanted witnesses to point out certain things, they pointed them out on my diagrams. They were key to the trial.”
In the end, Dzhokar Tsarnaev was found guilty of multiple counts, including conspiracy to use a weapon of mass destruction in the killing of three people and the injuring of 264 others at the Boston Marathon finish line on April 15, 2013. He was formally sentenced to death on June 24, 2015.
On May 18, 2016, U. S. Attorney Carmen Ortiz presented Dowd with the Department of Justice’s Investigative Achievement Award in recognition of his contribution to the United States v. Dzhokhar Tsarnaev.