WASHINGTON, March 3 (Reuters) - U.S. lawmakers facing a possible vote on a free trade pact with Colombia need better information about why a highly respected Colombian judge was removed from a panel handling cases involving the murders of trade unionists, a senior U.S. Democrat said on Monday.

Rep. George Miller, chairman of the House of Representatives Education and Labor Committee, pressed the issue in a letter to the administrative arm of Colombia's independent judicial branch, after Colombia's vice president said Miller's questions were best directed to them.

Miller expressed concern that Colombian Judge Jose Nirio Sanchez may have been removed from the three-judge panel in retaliation for his rulings in a high-profile murder case involving Colombia's armed forces and for ordering a probe into whether Nestle SA had a role in another labor killing case.

"As the U.S. Congress debates the proposed trade agreement with Colombia, it is essential that we consider whether Colombia is doing everything it can to protect the safety of workers who want to exercise their basic workplace rights without worrying about becoming a victim of violence," Miller said in a statement.

"Colombia must have an independent, effective, sustainable process for investigating, prosecuting, and adjudicating these cases. If it lacks such a process, then killers will continue to murder union members with impunity," Miller said.

Miller is a close ally of House Speaker Nancy Pelosi, who has insisted that Colombia make more progress in stopping the killings of trade unionists and putting their murderers in jail before Congress votes on the free trade agreement.

Miller wrote Colombian President Alvaro Uribe in January to first ask why Sanchez had been removed.

In a reply dated Feb. 13, Colombian Vice President Francisco Santos said the decision had been made by Colombia's independent judicial branch. "Thus, many of the answers to the questions you raised lie with them," Santos said.

However, Santos said Sanchez was part of a panel set up to work on a backlog of cases, and Colombia law requires such judges to be renewed every six months.

"While Judge Sanchez has an excellent reputation, so does his successor Judge (Teresa) Castillo de Casa, who has served for 19 years with distinction," Santos said.

The Colombian government has already made significant progress toward eradicating violence and impunity, and will be "unwavering" in its efforts to do that, Santos said. (Editing by Eric Walsh)

AlertNet news is provided by