The number of LNG tankers steaming through Boston Harbor under heavy guard has dramatically increased since 2001 - with shipments of the dangerous cargo now averaging more than one a week, according to data reviewed by the Herald.

There were 63 shipments of liquefied natural gas through the harbor last year, up 61 percent from 2001 and up nearly 40 percent since 2000, according to Department of Energy records. Shipments were disrupted for a few months in 2001 after the Sept. 11 terrorist attacks raised fears of an incident in Boston.

There were 67 LNG shipments in 2004 - an increase of about 72 percent over 2001's disrupted schedule and a jump of 49 percent over 2000.

Shipments over the past two years have averaged about one every 5.6 days, compared with one every eight days in the two years before Sept. 11.

"We're certainly concerned," said James Hunt, Mayor Thomas M. Menino's environment and energy chief.

Julie Vitek, a spokesman for Distrigas, which runs the terminal, said the increase in shipments is largely due to the huge demand for natural gas by Everett's Mystic Generating Station, which in 2003 switched from burning coal to using gas to fire generators.

Nonetheless, Hunt said the spike in deliveries - which now require a small army of security - is one of the reasons Menino wants to put a cap on tankers using the harbor.

Eric Poulin, a spokesman for Fall River Mayor Edward Lambert, who is bitterly opposed to a planned new LNG facility in that city, said officials there are worried about a similar upward trend in shipments.

He noted that Weaver's Cove LLC, which is pushing for the Fall River facility, recently revised its application to increase proposed shipments from about 60 per year to as many as 120 shipments. "Once they're approved, they can bring in as many as they want," said Poulin.

Vitek, however, said Distrigas has been completely open about increased LNG shipments. While the company, in 2001, was saying the shipments came in about every 10 days, Distrigas now says the tankers arrive about once a week.