"5 min after" vs. "at 5 min after"

There is nothing wrong with omitting "at" in the example you cited. "5 min after" sufficiently defines the time. Also, the report obviously uses an abbreviated style; orherwise it would have spelled out "minutes" ( or at least "mins").

Anyway, In AmE, we tend to reserve the "at" in time references for instances where we cite a specific time on the clock:

• The meeting will resume at ten after two.

When speaking of relative time (elapsed time between two events, without reference to the clock) we typically do not use "at", and in many cases it would strike us odd to hear it that way.

• He fell asleep ten minutes after he got here.

not

• He fell asleep at ten minutes after he got here.

However, in your sample, the experimenters are figuratively "starting a clock" (a virtual stopwatch") counting off the minutes. So although "at" is not needed, it doesn't sound strange to leave it in, in that specific context.