Multiple Unknown Username Login Attempts for Event ID 4625. Is there a way to narrow down the source?

We've identified an obscene amount of user login attempts from users that are not in our domain. Below are example log files. We can't narrow down where these logins are coming from. None of the computer sources are showing, unfortunately.

Is there another way I can narrow down the source of these logins?

ERROR EX 1

An account failed to log on.

Subject:    Security ID:        NULL SID    Account Name:       -   Account Domain:     -
    Logon ID:       0x0

Logon Type:         3

Account For Which Logon Failed:     Security ID:        NULL SID    Account
Name:       ITUSER  Account Domain:     

Failure Information:    Failure Reason:     Unknown user name or bad
password.   Status:         0xC000006D  Sub Status:     0xC0000064

Process Information:    Caller Process ID:  0x0     Caller Process Name:    -

Network Information:    Workstation Name:       Source Network Address: -
    Source Port:        -

Detailed Authentication Information:    Logon Process:      NtLmSsp 
    Authentication Package: NTLM    Transited Services: -   Package Name
(NTLM only):    -   Key Length:     0

This event is generated when a logon request fails. It is generated on
the computer where access was attempted.

The Subject fields indicate the account on the local system which
requested the logon. This is most commonly a service such as the
Server service, or a local process such as Winlogon.exe or
Services.exe.

The Logon Type field indicates the kind of logon that was requested.
The most common types are 2 (interactive) and 3 (network).

The Process Information fields indicate which account and process on
the system requested the logon.

The Network Information fields indicate where a remote logon request
originated. Workstation name is not always available and may be left
blank in some cases.

The authentication information fields provide detailed information
about this specific logon request.
    - Transited services indicate which intermediate services have participated in this logon request.
    - Package name indicates which sub-protocol was used among the NTLM protocols.
    - Key length indicates the length of the generated session key. This will be 0 if no session key was requested.

ERROR EX 2

An account failed to log on.

Subject:    Security ID:        NULL SID    Account Name:       -   Account Domain:     -
    Logon ID:       0x0

Logon Type:         3

Account For Which Logon Failed:     Security ID:        NULL SID    Account
Name:       LURRUTI     Account Domain:     

Failure Information:    Failure Reason:     Unknown user name or bad
password.   Status:         0xC000006D  Sub Status:     0xC0000064

Process Information:    Caller Process ID:  0x0     Caller Process Name:    -

Network Information:    Workstation Name:       Source Network Address: -
    Source Port:        -

Detailed Authentication Information:    Logon Process:      NtLmSsp 
    Authentication Package: NTLM    Transited Services: -   Package Name
(NTLM only):    -   Key Length:     0

This event is generated when a logon request fails. It is generated on
the computer where access was attempted.

The Subject fields indicate the account on the local system which
requested the logon. This is most commonly a service such as the
Server service, or a local process such as Winlogon.exe or
Services.exe.

The Logon Type field indicates the kind of logon that was requested.
The most common types are 2 (interactive) and 3 (network).

The Process Information fields indicate which account and process on
the system requested the logon.

The Network Information fields indicate where a remote logon request
originated. Workstation name is not always available and may be left
blank in some cases.

The authentication information fields provide detailed information
about this specific logon request.
    - Transited services indicate which intermediate services have participated in this logon request.
    - Package name indicates which sub-protocol was used among the NTLM protocols.
    - Key length indicates the length of the generated session key. This will be 0 if no session key was requested.