BEIRUT -- The head of the international tribunal on the assassination of Lebanon's former prime minister sought to reassure Lebanese this week that the investigation is on track, but there are growing concerns here that work is languishing in the case.

For supporters of the slain former Prime Minister Rafik Hariri, the court is their key to hopes for uncovering who was behind the February 2005 suicide truck bombing that killed him. Many Lebanese accuse neighboring Syria.

Syria denies any involvement, but the killing led to the withdrawal of Syrian troops and the end of Damascus' 29-year domination of the country. That opened the door to a still unresolved struggle for power between Syrian-backed Lebanese led by Hezbollah and pro-Western factions.

Hariri's supporters and their allies are preparing for a mass rally in downtown Beirut on Feb. 14 to mark the fifth anniversary of Hariri's assassination. In the past years hundreds of thousands of people took part in the rally.

The Netherlands-based Special Tribunal for Lebanon was formed one year ago after years of investigations. But progress in starting trials has been slow - and it is still unknown who might be charged in the case.

Two high-level departures from the court in recent weeks have increased the worries of Hariri's backers.

In January, the court announced that its chief administrator, David Tolbert, was stepping down to lead a New York human rights group and that its chief of investigation was leaving at the end of his contract in February to resume his duties as a police chief in Australia.

Tolbert's predecessor resigned four months after the tribunal was inaugurated.

The court's president, Antonio Cassese, began his first visit to Lebanon on Monday and for the past week has been briefing top officials on the case.

"The tribunal is alive and very healthy," Cassese said in an interview with the As-Safir newspaper published Friday. "We are dealing with an extremely complicated case that has to do with a terrorist crime, one that is a precedent of its kind in the field of international justice."

He said the two departures were "very normal" and "cannot affect the work of the tribunal because it (the tribunal) is not based on individuals but on teams that are very much capable and professional."

But some in Lebanon are not convinced.

"The Lebanon tribunal is not yet dead, but it seems very nearly there, amid embarrassing indifference in Beirut," Michael Young, an opinion writer for Lebanon's English-language newspaper the Daily Star, wrote on Jan. 14.

"Those committed to the rights of the victims must denounce more forcefully the charade now taking place in a suburb of The Hague. The supreme insult is to be told that justice will come when everything points to the contrary," he said.

Sari Hanafi, a professor who teaches transitional justice at the American University of Beirut, said some within Hariri's circle have expressed concern that the improving ties between Syria and its Western and Arab rivals could lead to an easing off of the court's work to prevent indictments that could inflame old tensions.

"There are concerns that the court could be an object for trade off," Hanafi said.

In December, Saad Hariri, the slain Hariri's son who accused Syria in the assassination and has since become prime minister, visited Damascus for the first time since the 2005 killing. Relations between the Hariri-led Western-backed coalition and Syrian-supported groups in Lebanon have been improving after years of tension that almost drove the country into a civil war.

The court denies any politicization of the tribunal.

The court prosecutor's spokeswoman, Radhia Achouri, said the tribunal acts "in total independence from politics and any other considerations."