respondiente

14:04 Apr 3, 2019 

Thank you very much for your responses!

06:03 Apr 2, 2019 

In Argentina "primer respondiente" doesn't necessarily refer to a police officer; it most often applies to emergency medical services. Here "cadena de custodia" clearly suggests that it means the first police officer to arrive. Parademics/ambulance staff may already be there but they have nothing to do with the chain of custody, of course.

Phil is right that the same is true of "first responder" in English (BrE too). It often doesn't refer to a police officer, but it can, and from the context I think "first responder" will be taken to mean first police responder, as in Spanish. The American term specifically for the first law enforcement officer at the scene is initial responding officer.
https://definedterm.com/initial_responding_officer/295151
https://www.nij.gov/topics/law-enforcement/investigations/cr...

In the UK you can refer to the "first officer on/at the scene", but "first responder" is a standard term in police contexts:
https://www.app.college.police.uk/app-content/major-investig...
https://www.scotland.police.uk/whats-happening/featured-arti...

20:52 Apr 1, 2019 

will depend on your overall context

20:49 Apr 1, 2019 

https://en.wikipedia.org/wiki/First_responder A first responder is a person with specialized training who is among the first to arrive and provide assistance at the scene of an emergency, such as an accident, natural disaster, or terrorist attack. ***First responders typically include paramedics, emergency medical technicians, police officers, firefighters, rescuers, and other trained members of organisations connected with this type of work. ***

20:48 Apr 1, 2019 

check out your specific context to see which term better applies

20:45 Apr 1, 2019 

I think it partly depends on the context. First responder in US English usually means any member of the emergency services, whereas in your context it may refer to the first police officer to arrive.

20:42 Apr 1, 2019 

first responder. noun. A person, such as a police officer, firefighter, or EMT, trained in urgent medical care and other emergency procedures and prepared to move quickly to the scene of an accident or disaster.