Someone pointed out earlier, if the failures are load bearing analysis will be needed on adjacent truss parts for closer inspection. I would like to think the bridge contractors already have the bridge design loaded into some type of analysis software that gives a ballpark figure for how bad it is. It will likely take a combination of computer analysis and visual inspections to come to a conclusion. Safety has to be priority one.
Ah yes “safety” the battle cry of leftists everywhere. You probably don't want to go there with me. Nothing is completely safe. The masks and lockdowns are a good example of safety at all costs. Risk is part of the human condition.
This part of the bridge was under stress obviously, but the trigger for the crack that resulted in a break was likely from an unusual amount of stress from factors such as settling in the bridge structure. This was probably exacerbated by a defect in the steel or some type of corrosion that made it through the green Zinc Oxide paint and created a weak area in a critical spot. These types of factors have a healthy dose of randomness that will rely more on visual inspection and testing than computer analysis.
Even though the paint they use on steel bridges always has a lot of zinc in it and the paint is designed for multiple applications it is typically more brittle than the steel. After 50 years places where the steel is flexing and are the same areas most likely to allow moisture to get into where it can cause problems.