Two days after six small underground explosions rocked downtown Baltimore, authorities remained stumped yesterday as to how 1,000 gallons of a highly flammable chemical pooled beneath a major intersection without notice before blowing up.

City, state and CSX Corp. officials used words such as "baffling," "puzzling" and "a mystery" as they pondered how tripropylene - commonly used by industry - ignited Saturday under Pratt and Light streets and blew a 300-pound manhole cover 4 feet into the air.

Investigators continued to theorize that the tripropylene entered the storm drain after leaking from a tanker car during last month's freight train derailment and fire in the Howard Street Tunnel, four blocks west of the intersection. But without anything definitive to go on, "we're just spinning our wheels," said Inspector Michael M. Maybin, a Fire Department spokesman.

Among the conundrums:

  • Firefighters responded to several complaints about noxious fumes in the area before last weekend but found no dangerous air readings.

  • Some officials speculate the chemical's journey began in the CSX-owned tunnel, but soil tests soon after the 18,000-gallon tripropylene tanker car was removed showed no trace of the substance.

  • Friday's rains are thought to have carried the floating chemical to the area beneath Pratt and Light, but officials note that tens of millions of gallons of water coursed through those same pipes during the fire.

    "As soon as you make a connection, you go, 'Oh, wait a minute,' and it doesn't hook up," said Robert H. Murrow, a city public works spokesman.

    Adding to the challenge for investigators is the city's labyrinthine network of storm-water pipes detailed in frayed, decades-old maps. A dye expected to be pumped into the pipes may help show how the pieces fit together.

    Yesterday, state environmental crews found residual amounts of tripropylene in a conduit carrying wires and cables. Crews worked last night to vacuum it out.

    This much was clear, said Maybin: About 6 a.m. Saturday, tripropylene vapor ignited and launched a manhole cover. Firefighters pulled other covers off, as two explosions followed.

    No one was hurt, but hours later, the incident slowed Orioles fans en route to a day game, knocked out traffic lights and hurt business at nearby shops. About 5 p.m., Maybin said, vapor ignited three more times in storm drains.

    Maybin said the department had received "several" complaints about fumes after the tunnel fire. Firefighters, backed up in some cases by state environmental officials, monitored the air repeatedly, officials said. Two devices - a gas meter and a photo-ionization detector - turned up no indication of a risk of explosion.

    "We had no reason to suspect anything was there, other than some lingering vapors," said fire Lt. Ronald Addison, hazardous-materials coordinator.

    Monitors failed to detect the tripropylene, Addison said, because it is heavier than air and lingered underground.

    Until Saturday, officials thought the tripropylene had burned off in the fire, despite a small hole found in the side of the tanker car. That view seemed bolstered by soil samples from the tunnel after the July 18 incident showing no trace of the chemical.

    Robert L. Gould, a CSX spokesman, said it was "extremely premature" to conclude that the chemical leaked from the tunnel into the storm drain.

    Tripropylene is a common industrial chemical. As many as 59 companies in Baltimore use it to make products ranging from insecticides to dry-cleaning soaps, said Richard McIntire, spokesman for the Maryland Department of the Environment. Many city storm drains lead to the harbor area.

    CSX has acknowledged no fault, but has offered to pay $75,000 to $100,000 to help with cleanup costs, unless the chemical can be traced to another source. That amount is in addition to nearly $500,000 for city overtime costs related to the tunnel fire, and $7,000 to 10 Howard Street merchants who suffered losses.

    Even if the chemical did seep from the tunnel, authorities wondered why it would not show up for weeks, after a rainstorm.

    "You had millions of gallons of water pouring through that system to help fight the train fire," McIntire said. "If that stuff was present in the system, then you should have seen it. It's another one of those puzzling questions."

    It doesn't take much to set tripropylene off; in some forms, its flash point - the temperature at which it can ignite - is 75 degrees.

    Fire and Baltimore Gas and Electric Co. officials said they doubt electric lines provided a spark; Maybin said three explosions were in a drain with no wires.