What causes an R script to get Killed?

I've been running some memory intensive processes on EC2 servers. The code runs quite well for about 12-14 hours (it's running 1000s of simulations on 12-14 large datasets) and then all of a sudden I just see the message "Killed" with no further explanation.

What makes R do that?

UPDATE: My server specs.

Why was my process killed Linux?

The Linux Kernel may also decide to terminate one or more processes when the system is running low on resources. A very common example of that is the out-of-memory (OOM) killer, which takes action when the system's physical memory is getting exhausted.

How do I know if a process is killed in Unix?

If the kernel killed a process (because the system ran out of memory), there will be a kernel log message. Check in /var/log/kern. log (on Debian/Ubuntu, other distributions might send kernel logs to a different file, but usually under /var/log under Linux).

It could be the out of memory killer of the operating system.

Are you cleaning up your workspace when you have finished with a dataset?