Row / Column Constraint
Each row can contain at most one suspect, and each column can contain at most one suspect.
Explanation
This is the 'sudoku' part of Murdoku. No two suspects can share a row or a column. This constraint is always active and is a powerful elimination tool: placing one suspect removes an entire row and column from all other suspects' candidates.
Example
If you place Max in row 3, column 4, no other suspect can be in row 3 or column 4.