I got the reply from blockchain.info as below. So they believe it's likely that my computer get infected by malware...
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I am sorry to hear of these transactions that you did not initiate. You may have some type of malware on your computer that resulted in your funds being stolen, because your private information was somehow obtained.
By design, Blockchain.info never has access to users' accounts or bitcoins, we do not control access to your funds. That is done on your computer only. We cannot reverse transactions, return your money or track the destination of the transfer made from your bitcoin address. Bitcoin transactions cannot be reversed or tracked.
Our level of encryption, and the complex password that you used, makes it highly highly highly unlikely this account was accessed by a brute-force attack.
If you keep your password and private key backups secure, then your bitcoins are safe with blockchain.info.
It's a terrible thing to lose your bitcoins. It really is. The best thing you can do at this point is to learn from what happened and make sure you take as many security precautions as you can afford to take (like the google 2FA, and IP-locking your account as well).
Again, sorry this happened to you
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Kevin Houk | Blockchain & ZeroBlock Support
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