Greping the string from a log

Hi Folks,

I am searching something in multiple logs , actually an even ocuurs which get recorded in logs with a special word so rite now I am opening an individual log in vi and then searching it but total in the directory there are such 15 logs and it is very difficult to go one by one so please advise how can I use the power of grep with case insensitive search let say the directory is

/opt/dfgr/logs

which contain 17 different log and I want to search the string started app it mite bhi in all logs or in few logs , but I want to do case insenetive search please advise

something like grep -i 'started app' /opt/dfgr/logs/*.log ?