By Adam Easton
BBC News, Warsaw
A European Parliament delegation probing allegations the CIA held terror suspects in illegal European detention centres has sharply attacked Poland.
The CIA is said to have detained terror suspects in secret jails
Poland was criticised for failing to co-operate with the investigation and warned it could lose voting rights in the EU if the allegations were proved.
MEPs met just one minor government official, plus airport management and journalists in three days in Warsaw.
Poland has repeatedly denied allegations it hosted a secret prison.
Other delegations have already visited the US, UK, Germany and Romania.
Poland was one of two countries in eastern Europe identified by the group Human Rights Watch as a possible location for a secret detention centre.
At the end of their visit, the delegation members said they had uncovered very little new evidence.
The MEPs said they had never before encountered such a lack of co-operation.
At least one witness they did interview gave inconsistent replies to their questions, the delegates added.
In the end there was no "smoking gun".
Instead, the Dutch MEP Sophie in't Veld told the BBC, they came up against a "wall of silence".
"I think that we all got the feeling that there is something going on, there is something they are trying to hide," she said.
"We do not know exactly what it is so we cannot prove that there has been a prison here but there have been definitely CIA activities here which apparently need to be covered up."
The MEPs were investigating claims by Human Rights Watch that CIA flights landed at a small airport in north-eastern Poland.
The airport is close to a base which houses the military's intelligence school.
Poland is a close ally of the US and the government admits its secret services collaborate with the Americans.
But after conducting its own inquiry the government says it is satisfied there were no secret prisons on Polish soil.
If that turns out not to be true, the delegation's chairman, Portuguese MEP Carlos Coelho, said Poland could end up losing its voting rights in the EU.