A blockchain bridge lets an asset that exists on one network, Bitcoin for example, be represented and used on a different network, such as Ethereum. It does this by locking the original asset on its home network and creating a corresponding token on the destination network that represents it.
Two ledgers, one movement of value
From an investigative standpoint, a bridge transaction creates two separate records on two separate ledgers that describe the same underlying movement of value. Reading only one side of that record gives an incomplete, and sometimes misleading, picture of what actually happened to the funds.
A legitimate tool that also adds friction to a trace
Bridges are used constantly for entirely legitimate reasons, moving liquidity, accessing a specific application, or simplifying a portfolio, but they are also a common step in attempts to obscure the origin of stolen funds, since each additional network adds friction to a straightforward trace and requires its own separate tooling to follow correctly.
Reputable bridge services keep transparent, publicly available transaction records, which is exactly what makes it possible to reconnect both sides of a movement even when a bridge is used deliberately to slow a trace down.
In practice, an investigation touching a bridge transaction means treating each side of the ledger as its own transaction to verify, then confirming the two are genuinely linked by amount, timing, and the bridge contract itself, rather than assuming the connection.