That finding overturns an earlier conclusion that placed the blame for the collision on Ms. Ward.
The report also did not blame the truck driver. The truck and bicycle came into contact "while the bike was still in an upright position," a CHP statement said.
The truck, which has an extended front end, had "very significant" blind spots for a "significant time that they were in proximity," CHP Capt. Mike Maskarish said in a telephone interview. "In this case, we just can't determine which party is more at fault."
After discovery of a "very, very minute" amount of DNA found under the left side of the truck cab near the front axle, the investigation by the CHP's Multidisciplinary Accident Investigation Team and a forensics team from the San Mateo County crime lab concluded that the rear tire of Ms. Ward's bike collided with the front bumper of the truck, Mr. Maskarish said.
Investigators concluded that the DNA was human, but because the sample was so small and because months of time had passed, it had become impossible to type the DNA and identify it as belonging to Ms. Ward, Mr. Maskarish said.
The investigation included a re-enactment/reconstruction of the accident, a task complicated by a lack of witnesses and of any physical evidence, not even paint flecks from Ms. Ward's pink bike, he said.
A massive chrome truck bumper and an insubstantial rubber bicycle tire "were not surfaces that would really lend themselves to a really good transfer (of materials)" in a collision, Mr. Maskarish said. "It's not going to take much for the rider to lose control."
The original speculation held that Ms. Ward had turned into the left side of the truck cab, perhaps because she got hemmed in by another vehicle. It is not uncommon for bikes and vehicles to be next to each other in the three lanes of this freeway underpass.
Cyclists run a 100-yard gauntlet that includes stark and sudden darkness when passing under I-280. In trying to regain the right side of the roadway headed west into Ladera, cyclists and drivers must negotiate two merges that cross each other: Vehicles at the stop sign may be aligned with the southbound freeway but want to go west, or vice versa.
"This is just such a horrific tragic accident," Mr. Maskarish added. "It's taken a tremendous amount of time to put this whole thing into place and get this reconstruction together. There has been a very high quality of attention that this department has put into this in fairness to everyone involved."
For the truck driver, Gabriel Manzur Vera, this was the third fatal accident involving his truck since 2003, but in none of them was he found to be at fault, CHP Officer Art Montiel said.
(Ms. Ward's relatives launched a wrongful death lawsuit in January against Mr. Vera and the trucking company, Randazzo Enterprises Inc. of Castroville.)
It seems an unusual number of fatal accidents. "I'm sure it's happened before but I'm not aware of it," Mr. Maskarish said. "It was a coincidence and nothing but that in this particular case."