Police continued Saturday to investigate an apparently deliberate hit-and-run Friday afternoon in Lowell that seriously injured a 15-year-old boy on a bicycle, according to authorities.

The boy, who suffered head and internal injuries, was in serious but stable condition at Boston Children’s Hospital midday Saturday, said Lowell police Captain Timothy Crowley. No arrests have been made.

Lowell police asked for the public’s help in identifying a white Chevrolet pickup truck allegedly used to strike the teenager about 2:30 p.m. on Lawrence Street.

The occupants of the truck reportedly had been speaking with the boy and a friend. After the discussion became heated, the friend fled on foot while the boy biked in the opposite direction. According to witnesses, the truck returned and crossed over into the wrong lane in order to strike the bicyclist before speeding away, said Crowley.

Police at the scene found only a wheel and the chain left from the bike. The rest of the bike was found about a mile away on Bolt Street, near the abandoned truck, said Crowley.

In a separate and apparently unrelated incident a few hours after the bicyclist was hit, a motor vehicle was used as a weapon again when a man allegedly injured his estranged wife’s boyfriend by driving at him and pinning him between cars, according to Lowell police.

Officers arrived on Caddell Avenue in Lowell at 5:25 p.m. Friday for a reported fight between two men and found the victim “bleeding profusely’’ from a serious leg injury, said Crowley in a prepared statement.

A preliminary investigation found that the 28-year-old victim and Andrew Melendez, also 28, had been involved in “some type of domestic-related altercation,’’ said Crowley.

Melendez was on the street to drop off his son at the home of his estranged wife, said Crowley.

Melendez got in an Acura RSX sedan while the victim was standing between it and the back of a parked Mazda SUV, said Crowley. Melendez then drove forward, pinning the victim between the two cars.

The victim’s girlfriend “probably saved him from bleeding out’’ by applying a tourniquet to stop the blood flow, said Crowley.

The victim was taken to Lowell General Hospital and then by helicopter to Brigham and Women’s Hospital in Boston.

Melendez, who is from Kittery, Maine, was charged with assault and battery by means of a deadly weapon and with armed assault with intent to kill.

Reenat Sinay contributed to this report. Nicole Fleming can be reached at nicole.fleming@ globe.com.