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what is sitting on USB device?

Trying to connect to a device, I get this error message:

*** Error ***
An error occurred in the io-library ('Could not claim the USB
device'): Could not claim interface 0 (Device or resource busy). Make
sure no other program (gvfs-gphoto2-volume-monitor) or kernel module
(such as sdc2xx, stv680, spca50x) is using the device and you have
read/write access to the device.
*** Error (-53: 'Could not claim the USB device') ***

On general Linux principles, how does one go about what is keeping the
device busy? How does one distinguish between "busy" and a permissions

I can see that the system detects the device by, say, lsusb:
bus 001 device 007 ... Nikon

I haven't got anywhere with "lsof", but that at that point the
specifics of this system may come into play (Debian 9 installed as a
Crouton target on a chromebook).

Any suggestions off-hand?