Currently they are only 'imputados', i.e. there are 'reasonable reasons' to believe that these persons could be directly connected to a criminal event.
On the other hand, being 'imputados' gives them additional procedural rights which they do not have when they are mere 'witnesses': you do not have to testify against yourself, you hav eaccess to all of the data in the judges possession...which in one sense may be better for them.
Which means that the judge is doing his job. The engineers have been notified of the investigation on their actions... The judge cannot discount criminal negligence without proper investigation... and according to the law he has to notify the parties about the ongoing investigation...