Federal investigators are in Torrance this week looking into a Feb. 18 explosion at the Exxon Mobil oil refinery.

Meanwhile, in Houston, union leaders and oil companies resumed negotiations to replace an expired labor contract that has resulted in strikes at 15 petroleum and chemical refineries nationwide.

Four or five investigators from the U.S. Chemical Safety Board, accompanied by a personal safety team, are scheduled Monday to begin their probe into what caused last month’s blast in Torrance.

The visit was sparked by a formal request sent to the board Friday by U.S. Reps. Ted Lieu (D-Torrance) and Maxine Waters (D-Los Angeles).

In their letter, Lieu and Waters point out that the Torrance accident was not an isolated incident. The facility suffered other explosions in 1988 and 1994 that together injured 37 people and killed one.

Last month’s blast was the third refinery explosion so far this year in the U.S. – an accident rate higher than in Europe, according to the letter from the members of Congress.

Exxon Mobil confirmed that investigators are reviewing the incident.

“We are working closely with state and federal agencies to determine the cause of the explosion,” said spokeswoman Gesuina Paras. “We are cooperating with authorities who are investigating the incident.”

Government officials have been keenly interested in the Exxon Mobil incident, which shook Torrance with the force of a 1.7 magnitude earthquake. Much of the surrounding area was blanketed in a white ash containing compounds that can irritate the skin, eyes and throat.

At a hearing last week conducted by state senators at Torrance City Hall, the refinery’s manager said the blast was caused by excess pressure in the electrostatic precipitator – a pollution-monitoring device.

Exxon Mobil’s investigation into the explosion is likely to take months, he said.

Lieu and Waters said in their letter that they want investigators to find out whether Exxon Mobil broke any laws or regulations and whether the company’s safety procedures were acceptable.

The lawmakers noted that hydrofluoric acid, a corrosive compound able to dissolve glass, was being stored near the blast site. The Torrance refinery, they said, is one of 50 in the U.S. that uses the acid as a catalyst in petroleum refining.

Lieu and Waters raised questions about the vulnerability of Exxon Mobil’s hydrofluoric acid storage system.

In a response Friday to Lieu and Waters’ letter, Rafael Moure-Eraso, chairman of the chemical safety board, said the agency has been concerned about the Torrance explosion since it occurred, but at the time “had few resources available to consider deployment of a CSB team,” he wrote.

The board has been accused in the past of moving too slowly on investigations. But Moure-Eraso wrote that it has made progress on other projects in recent weeks and would send a team to Torrance.

Also Monday, efforts to renew a labor pact covering some 30,000 refinery workers continued.

The United Steelworkers union began calling for work stoppages in early February after the last contract expired and now has 6,500 refinery employees on strike.

Two of the 15 facilities affected by the walkout are in California – one in Carson and another in Martinez, both owned by Tesoro Corp.

After an extended stalemate and a teleconference meeting last week, the union is meeting face-to-face Monday with Shell Oil Co., the head negotiators for the affected oil companies, according to Shell spokesman Cameron Yost.